Gale certainly has dialogue where he expresses bitterness towards all the Gods. He also has dialogue available after his act 2 romance scene where he says he doesn't think he mattered much to Mystra: "I was not the first wizard to fall under her spell, nor will I be the last. I was an amusement to her, a mortal to be trifled with, amused, and eventually discarded." His next line is him acknowledging that he hurt her and saying he regrets it, so I don't think that interaction counts as him calling her a "crazy ex."
Upon further reflection, I have come up with further additional difficulties for a Gale sacrifice to be a satisfying good ending. The "man sacrifices himself to find redemption" thing is not unique. I've seen complaints that it's a trope. I'm not saying it can't be done well, it can, but there's competition. Of the top of my head, I can think of
Guardians of the Galaxy Vol. 2 and Return of the Jedi.
It would have to be written very well to compare favourably.
KillerRabbit has already pointed out the other big problem. In that situation, the story stops being about the player character and focuses on Gale.
I also remembered that Gale has an "eager to die" flag, so we can add that on to my list for proof of depression. I do think that interpreting Gale as depressed helps make his human endings more satisfying. Not chasing external validation anymore is great, and recovering from depression on top of that is lovely.
