It makes sense that the relationship between Gamora and Star-Lord was more open in Infinity War than in the Guardians films too based on the fact that it takes place four years after the battle with Ego. A lot can happen in that time, so it's only logical that relationship has grown. While we've been waiting to see the kiss for some time, what's arguably more intimate and meaningful is what Gamora entrusts Quill with doing for her. Making him promise to kill her to prevent Thanos from getting her is incredibly emotional and tells us a lot about their relationship. Quill listens to her and, despite his objections, loves her enough to commit to it. When the moment of truth comes, while he initially balks, he loves her enough to carry out her wishes. But it's ultimately all for naught, leading to Gamora's death and Quill making a grave mistake in his rage.