I have a plan for an arranged marriage between a Griffin Princess (Gilda) and a Changeling King (or earl at least) Gilda is fully aware of it and at first unhappy. How would the Changelings make her happy and show that they value her?

Perhaps by spinning a tale of others before that have done similar things. Showing Gilda that it wasn't all her fault while making sure she knows she should've handled things better. Being understanding would be a big help.

That's a very interesting question. This could be done many ways. She could be given a happiness spell, or Gilda could be shown that this would help combine two superpowers, which would benefit both countries.

Take on any form of a lover she wants to try out, tell her spy stories, do not nibble on her too hard.

Turn her into a griffon changling? No. Learn to cook? Maybe. Allow her to do whatever she wants? Most likely.