There always seemed to be a bit of a mismatch between how people depict Gale's confidence in fanworks and how he presents himself in-game to me?

One of his very first interactions at camp he does, admittedly cheekily, play off his Mirror Image as fastidious grooming habits, and dismisses the spell with a sly little "Handsome devil, isn't he?" It's playful and could be read as a bluff, but it does exist and doesn't feel particularly misplaced. This is a man that fucked God. I think that is great assurance that you must on some level be looking pretty good on the regular.

He also thinks he's charming, which he is! He has a line somewhere about having a dazzling smile. He calls Karlach "rough around the edges" but that he supposed he could be "smooth enough for the both of them." He is entirely confident that he can befriend Minthara and is the only one to even think to attempt it.

He is not a shy lover, either, once you are in private. Some of his lines are straight up scandalous. He may be a bit shy and bluster when you're the one to bring it up, but later on he is shameless. The "For now, I'll keep my mouth shut. Unless you have use for it, of course" line made my jaw drop. ALSO when you ask Gale if you were his first aside from Mystra, he says no, you are not. But you are the first AFTER Mystra. In fact, Loroakkan insults Gale by implying it was NOT by magical merit that he gained her favor, and all that merit did was insure he lost it.

Gale's insecurity is about his sense of self worth, and how the metric he judges his own worthiness is by what he can do, not who he is.