I don't see it ever happening. Gale's a very monogamous guy and there's a disconnect between surface and Underdark values that prevent Aldiirn feeling anything romantic for him. He's pretty decent for a topsider but doesn't really -get- what goes on in the Underdark. Two partners would also be overwhelming for Aldiirn since he hadn't had any longterm relationships before Astarion.

Maaayyyybe Astarion would be for it, but with my setup he sees Gale as a rival and I think he would rather shove winning Aldiirn in Gale's face lmao.