Thor, after getting rescued from the ship explosion in Avengers: Infinity War, informs the Guardians that Thanos is after all the Infinity Stones.
He also states that nobody knows where the Soul Stone is, so he can’t get them all.
Problem is, Gamora knows where the Soul Stone is. And we find out later that she’s told Nebula about it too. At this point Gamora should’ve realized she puts the whole universe in jeopardy by getting anywhere near Thanos.
She does ask Quill to kill her if she gets captured but this is a massive tactical error (IMO).