Btw, I suspect Gale is likely to be pretty flexible when it comes to both romantic and sexual configurations. We know that he was the lover of a certain famously polyamorous goddess, so at the very least he's presumably open to that. And while I've only tried his "spending the night" scene with a female PC (and the actual sex scene wasn't present in EA, just the prelude and aftermath), it's presumably similar for a male PC and he talks about working through what sounds like a Faerunian equivalent of the Kama Sutra so when it comes to positions he might be up (or down!) for anything.
Well yeah he’d be pretty hypocritical to say no to you bringing a third given his long game is a throuple between you, himself, and Mystra.