Friendly reminder to Galemancers that Gale's ambition is not something new.
In fact, I'd say Ambition is his "bread and bloody butter" (to quote God!Gale addressing Raphael)
(also, Mystra can bitch and moan all she wants, she inspires Ambition in her followers, this isn't unique to Gale but it certainly is one of his most dangerous traits. After all, when a mortal becomes a god, your godhood embodies what you are at your most base form. Gale becoming potentially God of Ambition is no fluke. It is Gale at his most base form)
Here's something we often forget when we write about Gale: the Gale we meet is heavily nerfed (twice, once by Orb, second by tadpole) and he's been humbled by his folly. Ambition is the last thing on is mind at the start of the game.
By the point you meet him, he was probably on his way north to find a secluded place to detonate the orb (Waterdeep is not one of the cities the Nautiloid passed, but Yartar is and that's to the north, hence the theory he was caught there on this way north) and he only starts to actively care about his survival when you pull him out of the waypoint sigil (he obviously doesn't want to die (his line from failing to help him out: "No! I'm slipping. I'll perish in here. I'll perish...")), he's more concerned for what the detonation of the orb will do than preserving his life since now he's not alone anymore.
Cue the orb reveal and him offering to leave. Again, he's more concerned about what the orb will do to others than him surviving. In his mind, he deserves this.
Cue meeting Elminster and Gale receiving his divine mandate: use the orb to destroy the heart of the Absolute and redeem himself in death.
Here's usually where many people fail to understand Gale's motives.
He resigns himself to use the orb because Mystra (his goddess, scrap the lover part, that's not even important here, his literally goddess whom he worships (and he does, there's literal lines in the game (by Wyll and Gale himself) confirming that Gale is a very religious man, his romance with Mystra was more than love, it was religious) has ordered him to do it. it has nothing to do with his residual feelings for her (but it does hurt him that she would not even tell him this order directly, that he means so little to her that she'd send his friend instead) and everything to do with religion (hence why it's so hard to convince him otherwise, he risks becoming Faithless... doubly so because he is one of her Chosen, even if formerly) and also Gale's intellect fucking him over here (he is a man of thought and reason, he's doing mental gymnastics to try and figure out a way that allows him to survive but unfortunately, he's at his lowest point here and he doesn't fight Mystra's demand, he caves to reason, that this is the best, quickest way to end the Absolute (it is but, I'd argue that Mystra's latest incarnation is the dumbest god in all of DnD because she failed to account for the countless souls that would be lost in the wake of the Absolute's death... soul death is no joke in the lore, Ao would probably deal with her harshly in the wake of Gale using the orb simply because of how badly she'd fuck up the material plane's supply of souls)
And then when you finally do convince him to not use the orb and Gale sees the Crown of Karsus, many assume he's becoming power hungry because he's learning to be selfish for once, or that this is just a wizard quirk, of wanting to understand dangerous artifacts, attain godhood etc. No, actually this is perfectly in character for Gale specifically.
See, Gale's goals shift according to how much information he has.
When the journey starts, it's merely survival, but as it progresses, he reasons that using the orb is the best path forward. if you romance him or befriend him, you can convince him not to use the orb and then his priorities shift again in the wake of Act 3's beginning. Fresh from surviving the Orb and Myrkul, he has a new lease on life. He's potentially in a romance and he's got his eyes set on the Crown. The Archmage is finally making his reappearance and it's up to the player to determine where Gale applies his ambition. And sometimes even with the player's input he can choose poorly (ala Sneaky God!Gale, which to me is actually the most narratively interesting ending because this is basically Gale deciding to become a god on his own and can have any number of valid headcanons to justify it)
Gale in Act 3 is the closest we get to seeing the Gale of Waterdeep from before the events of BG3 and him getting the Orb.
One does not become an Archmage by twiddling one's thumbs, you become an Archmage by being ruthless in your ambitions, by being the most intelligent person in the room at all times, by striving constantly for the next discovery, the next adventure etc... it isn't something you stumble into, you have to work very, very hard for it, relentlessly even.
Gale was already ambitious before the events of BG3, the orb and his romance with Mystra are testimony to that, so it stands to reason that ambition is a major part of him.
Even in his Professor!Gale ending, his ambition has shifted again, this time being either for his family (Morena and Tara) or his lover and his family. In the God!Gale ending, his ambition has reached its zenith and transcended (he's literally Ambition now) and he's sharing his Ambition with his petitioners.
Long story short, Gale's Ambition didn't just come out of nowhere, him becoming a god or professor or adventurer etc is not out of character (this is one of the reasons i love him so much, there's no bad ending for him besides using the Orb (and dying during apotheosis, but really that's more of a skill issue on Gale than anything since you'd think he'd do his research lol even without player input), he's always been ambitious.
The Gale we meet has been humbled and nerfed twice but the Archmage is still in there. He only starts to properly rear his head once he sees the Crown and survives Act 2...