Most ponies like being in control.
In fact, one might say that desire to be in control is a defining characteristic of ponies, and most ponies would quantify the difference between 'most' and 'all' in the previous statement as, effectively, a rounding error.
Pinkie Pie, as most ponies can tell you, is not like most ponies.
So, when Twilight Sparkle told her that she is the one who needs to improve her attitude?
It is difficult, even for her, to give up that vengeful desire to blast Gilda with a magical friendship laser to the skull and wipe her mind to something more palatable for the rest of ponykind. In the blink of an eye she works through the ethical conundrum when one puts the question like that (answer: yes, it's okay, because if it's good enough for Princess Luna then it's good enough for anypony) she has to think through what she will say to convince the rest of her friends, the Elements of Harmony.
Applejack will likely agree with her, especially if she is able to see firsthoof the low, despicable, mean tricks Gilda used earlier.
Rarity would do it once she sees how selfish Gilda is.
Twilight Sparkle is a little less likely, given that Twilight just told Pinkie Pie that she needed to change her attitude. But she might if Pinkie Pie just happens to mention what it would be like to get more empirical data on how the Elements of Harmony affect creatures, specifically non-ponies.
Fluttershy is probably unlikely to want to do it, but she might if knew how mean and nasty Gilda is.
Rainbow Dash would defend her friend, and be unwilling to help, especially since she was there to observe Gilda's meanness.
That all changes when Pinkie Pie witnesses Gilda harass her herdmate in the market.
The actions most foul serves to double Pinkie Pie's thirst for justice! It lights a fire in her belly like the Fire Sauce lights a fire in a pony's mane! Ooh, she should get herself some Fire Sauce and drink it straight down! Though that craving could have been her hankering for some fizzy drinks. Determined to make up her mind, Pinkie Pie goes straight to the one place she can always count on, to treat that yearning for treats!
Pinkie Pie finds herself sitting at a table in Sugarcube Corner, faster than most mares can make the connection between thought and action, staring down a cake that deserves to be devoured. It probably called somepony's dam an old tart, after all. She licks her lips, opening her mouth wide as she debates the merits of savoring this or devouring it in one go.
It was no contest, really.
Pinkie Pie rubs her slightly distended tummy; the cake has to go somewhere, after all! She giggles; she looks like she did when she was carrying Lemon! Ooh, lemons! She should get some lemons!
Since scarfing that sweet slab of sugar sufficiently slaked her sense of suffering, Pinkie Pie realized something that no mare ever wants to hear, especially when it comes from their stallion.
Maybe she was just hangry.
One dejected sigh later and Pinkie Pie perks up as she realizes: maybe that means Gilda is just hangry, too! She can't get her favorite food (pony) here in Equestria, so she has to make do with her fourth favorite food (fowl, specifically chicken). Pinkie Pie starts diagramming the various scenarios and possibilities on the nearest available surface, which just by coincidence happens to be a cake. Maybe if she throws Gilda a sweet party, she'll show that showoff how sweet sugar is! And Gilda will happily have cake, and then eat the cake, and they'll all be happy!
Which leads us back to why Pinkie Pie has to give up control, not just her earlier desire for sparkly rainbow rainbows to help Gilda see the light.
She has to delegate to Rainbow Dash to get the pranks done, rather than just push the time of the party back until after dinner. Which, given that she likes to be surprised, isn't actually all that hard to convince herself to do. In fact, it is downright easy, because she'll be surprised too! She probably could have just figured that Rainbow would have pulled off the pranks on her own, but it didn't hurt to have a bit of surety that this part of the party was performed.
And there need to be pranks because she knows Gilda digs a good prank, and she can plant a prank like a Diamond Dog digs a hole. And by pranking Gilda and everypony at the party, that grumpy griffon is sure to feel included!
Her tiny icing sculptures of a griffon and pony dancing are forgotten as Pinkie Pie scarfs down her chocolate canvas. An emergency party, though? Well, a surprise party is right out, because maybe Gilda is quite flighty and she can't (shouldn't?) cause a confrontation. Or worse, scare her off, or give her the excuse that she didn't know about the party. And she would need to invite her friends to come, and they would be able to show off all the aspects of what being a friend is really about!
She gets a few cupcakes cooking before she begins traipsing through town. Why is everypony always hiding in the last place she looks for them? Pinkie Pie ponders this perplexing puzzler prior to persuading a particular pair of partners. After Lyra, Bon Bon, the flower trio, and a host of other ponies Pinkie Pie pronks to Sweet Apple Acres, meeting up with Doug and Fluttershy (good she's okay) and Applejack, snagging the purple bottle out of Doug's stash.
Pinkie Pie pronks through Ponyville, her first stop the schoolhouse. Class is just letting out for the day, so she merely announces, "Attention, Everypony! There is a 'Greetings, Gilda!' Party at Sugarcube Corner in Fifteen Minutes! Tell Your Friends!" The fillies glance excitedly at each other before dashing off, spreading the word. Pinkie Pie nabs Lemon before her filly can canter off with the rest, flipping her to her back and taking off as she says, "Lemon, gonna need you to get a few games out and watch the door. Make sure everypony knows to welcome Gilda."
"Okie!"
The two return to Sugarcube Corner, depositing Lemon by the door as Pinkie Pie goes into the kitchen, a mischievous look in her eyes. She returns with a tray of cupcakes, but everypony would be suspecting that. Pinkie Pie grins; no one would expect a carrot to light them on fire! One generous dipping later, and Pinkie Pie has her first trap prank set.
Her next stop is the punch bowl. Hmm; it would get too predictable if everything at the party lit them on fire. But the punch could be sour apple instead of regular apple juice in the mix! Or, since she has them on hoof, a bunch of lemons added. Meh, works as well. She waves at Rainbow Dash; the pegasus places a few presents on the table, waves back, and then flies outside and into the sky.
Wait, isn't she delegating the pranks to Rainbow Dash?
Pinkie Pie gasps as she realizes! She'll be doubling up on the pranks! Oh no! Wait. Oh yes? Before Pinkie Pie can resolve the issue of whether or not there can be too many pranks at a party (answer: no) the first group of party-goers arrive.
Pinkie Pie pronks to the front door of Sugarcube Corner, exclaiming, "Welcome! Welcome! Glad to see you!" as Lemon directs them to a few hastily set up games. She internally grimaces; maybe she should have given herself a little bit more leeway, but thanks to Rainbow Dash the whole party got set up just in the nick of time!
"Howdy, Pinkie Pie," Applejack says, glancing back towards Sweet Apple Acres. "You sure got this party set up lickety-quick."
"It's for the friend of a friend," Pinkie Pie says as nicely as she can with her teeth tightly clenched together. "So it better go well!"
"Um, okay, Pinkie Pie," Applejack says, raising an eyebrow. "Something going on Ah'm not aware of?"
Pinkie Pie growls, her frustration coming through in her voice, "You could fill a book. A lot of books. With things going on that you aren't aware of." She perks right back up, "But as long as you don't touch, eat, or say anything, you'll be just fine!"
"Right." Applejack thinks to herself for a few seconds before looking around, "Ah'll just go... stand in this here corner." She starts to walk off before turning to Pinkie Pie, "Oh, and thanks for throwing this party, Pinkie Pie. Ah'm sure it means a lot to Rainbow that you're trying to become friends with her friend."
Pinkie Pie beams, "Of course! I'm here to put a smile on everypony's muzzle!" She drops down, rubbing her hooves together, menacingly saying, "Even if they aren't a pony, I'm still gonna!"
"What was that now?" Applejack says before shaking her head. Pinkie Pie gets ready to say something as Applejack continues, "Actually, nevermind. Oh, and Pinkie Pie?"
"Yes?"
Applejack glances at the table of food, "Ah don't suppose you mind getting me a plate of snacks that won't, um, surprise me?" She chuckles to herself, "Ah've had a long day, and Ah'll probably have to work through dinner to keep up with Big Mac."
"Sure thing!" Pinkie Pie pronks away as Applejack stands in one of the corners, (hopefully) perfectly safe from all the upcoming shenanigans. The bell chimes as more ponies enter the store, the party starting to get a little more spirited. "Welcome!" Pinkie Pie says, grinning at Rarity.
"So, who is this Gilda the party is for?" Rarity says as she walks over to Applejack. She smiles at Lemon as the filly brings her a small plate of cucumber slices smothered in cheese.
"Griffon friend of Rainbow's, Ah think," Applejack replies, eyeing the cheesy treat. "Those any good?"
Rarity levitates one of the slices up, eyeing the cheese suspiciously. "Peppers in the cheese, if I had to guess." She nibbles at the topping, her eyes lightly scrunching up as her face falls. "Yup. Tolerable, I suppose." She takes another small bite, the plate levitating to Applejack.
Twilight Sparkle and Spike enter next, shortly followed by Doug and Fluttershy. Doug heads to the snack table as Twilight spots Fluttershy, moving up and asking, "So, Fluttershy, you've met Gilda, right? What's she like?"
"Oh, um," Fluttershy stammers, glancing over as Doug yelps at the drink table, the splash of liquid on the floor. "Well, I'll tell you later, Twilight." The two awkwardly stare at each other for a few seconds, Twilight clearing her throat before she grimaces at the treat Applejack and Rarity are scarfing down.
Pinkie Pie beams, quickly pronking to the entrance as Rainbow Dash and Gilda crash into the party. Rainbow Dash, her eyes rolling in her upside down position on the floor, yells, "We'll get that trick next time, Gilda! Maybe with three of you it'll work!"
"Gilda!" Pinkie Pie exclaims, offering a hoof to the downed griffon. "Welcome to one of my signature Pinkie Pie parties! I really, sincerely, truly hope you feel welcome among us pony folk!"
Gilda stares at the outstretched limb for a few seconds before grunting, reaching up and grabbing onto Pinkie Pie's hoof.
They'll probably have to leave the door open, so the smell of charred feathers can blow out.
"Oh, Pinkie!" Rainbow Dash laughs, "The old hoof-shake buzzer! You are a scream!"
"Yeah," Gilda glowers as she brushes a few burnt feathers off of her, "uh, good one, Pinkie Pie."
Rainbow Dash smiles, "Come on, G! Let me introduce you to a couple of my friends!"
"Right behind you, Dash," Gilda says, stopping to glare at Pinkie Pie, the pony smiling right back.
Rainbow Dash walks over to Applejack and Rarity, motioning at the empty space behind her as she says, "Hey guys, this is Gilda!" She turns, a bit of a frown as she sees Pinkie Pie and Gilda chatting. Her smile quickly returns, "She's my griffon friend from Junior Speedsters!"
"She's... quite the character," Applejack says, nibbling on the last of Rarity's cucumbers.
Pinkie Pie breaks away from Gilda, heading towards Rainbow and pulling out a stick complete with marshmallow. Gilda turns her head to watch as Doug pants, half a lemon drop in his hand. He shakes his head, blinking rapidly as he says, "Um, Pinkie?" He holds up a glass of punch, his thumb and forefinger awkwardly placed around the bottom. "I think your glasses are defective, they're dripping." He demonstrates, briefly removing his finger and releasing a stream of punch out of the bottom of the glass. He takes a sip, his fingers keeping the glass from dripping, his mouth puckering up. He forces out, "New flavor?" as he glances to Applejack.
"Doug!" Rainbow loudly whispers, pulling him down to her level, "Stop it! You're ruining my pranks! Go sit in the corner with Applejack!"
"Oops!" Doug whispers back, trying to contort his mouth to an apologetic look, "Sorry!" He walks over to Applejack and Rarity, letting Applejack try a sip of the punch that she immediately spits out.
"This is lemon?!" Applejack harshly whispers, stomping her hoof. "Who-"
"Yes?" Lemon replies as she skips over, beaming at Applejack. "You need anything?"
"Not you!" Applejack snorts, pointing a hoof at Doug's glass, the human continuing to drink it. "What Ah need is a glass of juice that ain't spiked!"
"Yes?" Spike says as he walks over. "You need something?"
"No!" Applejack glares, Spike shrugging and walking away. Applejack turns to Lemon and says, "Okay, maybe just a glass of water to wash that taste out of my mouth. This here mouth is reserved for apples only!" She winks up at Doug, pushing her cheek with her tongue.
Doug rolls his eyes as Lemon says, "Coming right up! You want just regular water, or some of the bubbly stuff?"
Applejack shakes her head, "Just regular water is fine for me. Ah don't need any of that sparkling nonsense."
"Yes?" Twilight Sparkle says, walking over.
"Oh, for Pete's sake." Applejack grunts, facehoofing.
Meanwhile, Gilda walks over to the table of food, eyeing each of the dishes like an eagle. The cupcakes are right out, and from overhearing Doug she knows the lemon drops are trapped, even if they look good and she likes vanilla. There has to be some way the ponies know which food is good and which is doctored. A wall-eyed pegasus grabs one of the carrots with a huge smile, so Gilda does the same, smirking as she chows down.
Pinkie Pie lifts her stick into the resulting column of belched flame, smiling at the nice char that results.
Rainbow Dash shouts, "G! The punch!" as she tries to stifle her laughter.
Gilda screams, "HOT!" as she flies to the punch bowl, ignoring the conveniently placed glasses and instead sticking her head straight in. The horribly sour punch completely fails to quench the fire in her throat; in fact, it makes the burning even worse as it spreads it around. She grabs the first glass she sees, the water from Lemon's back also failing to do anything to help, acting much like the lemon spiked punch.
"Priceless!" Rainbow Dash forces out between her laughter, grinning as Gilda tries to keep her cool.
"Yeah. Priceless," Gilda deadpans. She moves up to the table laden with presents as Apple Bloom, Sweetie Belle, and Scootaloo join the party. Gilda snorts as Scootaloo stops dead in her tracks, grabbing one of the gifts while pointing a talon at the frozen filly. "Oh, it's the littlest dweeb! You want a present?"
"Hey!" exclaims Rainbow Dash, but she holds back from anything else as Gilda advances on Scootaloo.
Scootaloo glances towards her sisters before looking back at Gilda, trying to appear courageous as her legs shake. She shies down, quivering as she barely is able to keep her eyes on the griffon.
"Hah!" Gilda laughs with a sneer in her eyes, holding the cylindrical present forward while drawing a talon along the side, cutting through the wrapping paper like it is nothing. "Don't worry, little dweeb, I'll-"
The present explodes with a bang, four green snakes shooting out of the can and pelting Scootaloo! The purple maned filly yelps, dropping down with her hooves covering her head and cowering, pleading, "Please don't eat me!"
Scootaloo glances up at the sound of laughter; Gilda's feathers have poofed up, giving the griffon a fluffy look, like Angel Bunny after a particularly long bath. She snorts, her fear forgotten as she joins the rest of the ponies in giggling at the sight.
"That's it, you little twerp!" Gilda shouts angrily, swiping a talon at Scootaloo, "You better shut it, right now!" She sneers as Scootaloo's laughter cuts off, reveling in the terrified expression, "Oh, you aren't so tough now, are you?"
"Hey, Gilda!" Rainbow Dash exclaims, hovering a foot in the air and pointing a hoof at the griffon. "Back off!"
Gilda snorts, "Come on, Dash, it's just a bit of fun with only the lamest filly ever at the lamest party ever! These pranks are nothing! It's time for us to bail on this lame-o scene!"
"Yeah? Well, that lamest filly happens to be my filly!" Rainbow Dash gets right in Gilda's face, her rainbow mane flipping to the side as she flies next to Scootaloo, nuzzling her filly. "So I guess that me Queen Lame-O, huh?" Scootaloo beams up at her dam, inspired by her dam.
Gilda snorts, "You? You got knocked up? You're joshing me, right, Dash? I thought you wanted to be a great flier!" She sneers, shaking her head. "Guess we all can't live the dream like we used to. And here I thought you were cool!"
"Yeah, well, I can still fly circles 'round you, G! And this isn't how I thought my old friend would treat my new friends, or my family!"
"Come on. Dash. This party? All these lame pranks? They had to be Pinkie Pie's idea! She been trying to make a fool out of me this whole time!"
"Well, yeah!" Pinkie Pie says, pronking up. "I thought by making you feel included, by treating you like everypony else, we could improve that attitude of yours!"
"Yeah!" adds Rainbow Dash, waving her hoof behind her at the congregating ponies. "And the rest of those lame pranks were my idea." At Gilda's stunned expression Rainbow continues, "So if you don't want to hang with us lame-o ponies here, well, maybe you should find some 'cool' friends someplace else."
Gilda stares blankly at Rainbow Dash before she snorts again. "Whatever. You used to be cool, Dash." She flips the eagle as she leaves the party, her midair spin blowing everypony back a few inches.
Rainbow Dash turns to the gathered ponies, "Hey, everypony. I'm sorry about Gilda. I didn't think she'd be like that."
"It's okay, Dash!" Pinkie Pie says, reaching out a hoof. Rainbow smiles as she reaches out a hoof of her own, embracing her herdmate.
It's a good thing Gilda left the door open, so the stench of burned feathers and hair could air out.