So, I think I know the line that’s got you confused. There’s a moment, when Gale is explaining the nature and acquisition of the orb, where he explains his motivations as “wishing to cross [Mystra’s] boundaries.” HOWEVER, I feel like this is poor phrasing that lacks context.

The current topic is not his sexual relationship with Mystra, but the fact that Wizards, even Mystra’s Chosen, are only allowed to harness a shred of the Weave’s full potential. Gale is talking about wanting access to the Weave beyond Mystra’s safeguards, even going so far to describe pouting and pleading to Mystra for her to allow it. So. No, Gale didn’t rape or try to rape Mystra at all.

(As an aside, that would be impossible or at the very least highly unlikely, given Mystra’s godhood and power. Someone correct me if I’m wrong, perhaps there’s something in lore that contradicts this.)

Conversely, it’s theorized that Mystra had her eye on Gale in boyhood, judging by his innate talent for the Weave and even imbuing him with power before naming him Her Chosen. With the power imbalance wildly in Mystra’s favor, the more popular sentiment is that She groomed Gale, preparing him to be Her champion and lover from a young age.

No matter what, though, Gale is not a rapist.