NTA.It’s your wedding, serve what you want.If your dietary restrictions were foods that your guests couldn’t eat, I’d agree with her– it’s important to be a good host!– but everyone can eat vegan food and if they don’t like it, so what? It’s one meal.
Gale is the biggest asshole (and a true nightmare), but your fiancé is a little bit, too (and the irony is that he is one because he’s desperately trying to avoid being one…).I know families can be tough to balance, but ultimately he’s marrying you and creating a new family, and your new family should be priority 1 unless you’re okay with being second fiddle to that drama all the time.Because people like that will find a way to insert themselves constantly through the rest of your lives if you don’t set clear boundaries.
I know from personal experience that boundaries can be REALLY hard for people raised in traditional southern families.My sweet southern fiancé and I went to some couples counseling sessions partly because of this same issue of him being extreme peacemaker, which is lovely in some ways but not healthy in others.A person simply cannot please everyone all the time, and just because they may have spent a lifetime modifying their own behavior/sacrificing for the needs of their family doesn’t mean it’s fair for them to expect their partners to always fall in line, too.
It’s been a long and sometimes difficult journey for my fiancé, for sure, but a very good one for his personal development.

Maybe your fiancé would benefit from some personal work in this regard, as well?.