In the movie Avengers: Endgame, we learn that to get the Soul Stone, one doesn't have to make the sacrifice but simply lose the one they love. A soul for a soul.
Natasha kills herself, yet Clint receives the Soul Stone. There is another question here that deals with just that: Why was this sacrifice sufficient?
In the movie Avengers: Infinity War, we see Thanos with Gamora on Vormir. As soon as Gamora realizes that Thanos truly loves her and is going to sacrifice her, she tries to kill herself. But Thanos makes her knife disappear.
Had Gamora killed herself, would Thanos have still received the Soul Stone? Why does he bother making the knife disappear?
EDIT: I am looking for in rules in the Cinematic Universe that allows one to get the stone.