Rainbow Dash wasn't a pony who did things halfway. She'd procrastinate and put off work sometimes, but when it was time to get down to business she did her job well and she did it fast. She wasn't in charge of the weather team for nothing, after all. This extended to when she dated, which ended up giving her a reputation of being somewhat overbearing. It was a reputation she accepted, though she preferred to describe herself as being so awesome that most ponies couldn't handle dating her. She was passionate, driven, and always up for a challenge.
Gilda, similarly, was someone who lived according to her passions. She did what she wanted, when she wanted, and to tartarus with anyone dumb enough to try and get in her way. Life was too short and she didn't have time for dweebs who were too weak to handle an intimidating griffon like her. She was strong, stubborn, and with a temper just waiting to be flared up.
After their second reunion in Griffonstone, it wasn't really a question of if they would try getting back together, so much as when and how much damage they would end up causing when two irresistible forces clashed with each other. Rainbow would go to visit her every so often, and every time she would come back furious and venting to anypony who would listen about how surly and rude Gilda had been. Back in Griffonstone, Gilda would similarly vent to Greta and a few of the other friends she'd been making about how annoying Rainbow visiting was when she was trying to get work done.
Things only got worse a few years later when Griffonstone was finally back on it's feet and Gilda finally left. She ended up coming to Ponyville in search of a house. Ponyville was, in her words; "A lame pony town, but it's cheaper and it's not as much of a hellhole as Griffonstone." It didn't take long for Rainbow to hear about Gilda being back in town, which led to a long and passionate argument where Rainbow Dash insisted she come and crash with her and Gilda refused on the grounds that she didn't need her charity. In the end however, with minor mediating by Twilight, Gilda agreed to stay with Rainbow Dash just long enough to find herself a better place to stay.
The first few weeks living together weren’t anything like those stupid romance novels Rarity liked to read. They fought the entire time. Twice they'd had guards-ponies in their house because a concerned neighbor thought they might come to blows. After that though, anytime the guards showed up at their door it was more about noise complaints from their shouting than fears they'd cause each other harm.
It seemed all but inevitable to everyone who saw them together that eventually one was going to go too far and a brawl would erupt between them. And they were right. No one quite knows what was said, who set off whom, or who threw the first blow. But the next morning, Gilda was nursing a painful-looking black eye and Dash had several deep scratches on various parts of her body.
Whatever had happened that night, it appeared to have a net positive influence on their relationship. Arguments had been deescalated to minor bickering, shouting and thrown objects were at an all time low, and neither had called each other Gizelda or Crash in weeks. Noise complaints remained as infrequent as ever, but the content of said noise was now of a markedly different persuasion. It wasn't a perfect relationship by any stretch, and it wasn't even the last time they fought. Every so often they would begin to fight and argue the way they used to before that night, but such events were rare and tended to deescalate themselves before long.
Gilda never did end up finding her own place, and eventually Rainbow Dash just built an extension to her cloud home to better accommodate her roommate. Neither went so far as to call each other anything more than friends or roommates, and they blew off any insinuations that they were anything more than that, but they were effectively inseparable from that point on.
It was a life long, passionate, infuriating, loving, hateful friendship.