They've done more than kiss before. There was some actual, honest-to-god fooling around when under the influence of the potion, though no clothes every really came off. But Gamzee considers this all a kind of re-set, a do-over, and he wants to make sure he does it right. So instead of just shoving his tongue in Tavros's mouth the way he'd gotten used to doing whenever he had the opportunity, he keeps the kiss relatively slobber free. Just sweet and soft, and then he pulls back a little, biting his lower lip.