I have now heard Gale's story in game, and I'm inclined to believe him. First, he did not attempt to usurp Mystra's power, that was a backstory to his attempt to unlock the hidden weave, how that piece of Netherese magic came to exist. Second his ambition to unlock Mystra's greater mysteries is a very natural one for a wizard to have and in itself not evil. Third, his attempt to win Mystra over was reckless, which is also a rather common flaw for a wizard. In that way he's the archetypal wizard.
Of course it's still possible he's lying, but if he's not, he's actually a rather interesting character. I'm inclined to play as him somewhen.
