I am not sure how friendship Gale works, but the romance version is - as far as I know - a point system. Your arguments either add to or take away from his ambition counter, so you can end up on the war path with Mystra, on the forgiveness route (which in an extreme case adds a desire for self sacrifice) or somewhere in the middle. He remembers your intentions and when you switch them suddenly, he will be rightfully confused. I think a similar system would have done Astarion's quest good. Currently you can have a very different act 2 experience with him and it does not influence his choices during the ritual in the slightest.
