Gamora is one of Thanos' "daughters," he kidnapped her from a planet he destroyed. He had to sacrifice something he loved to receive the soul stone. The only thing he loved was Gamora. This is from inside the soul stone, I believe. She is represented as the child he kidnapped asking him, "did he do it?" Meaning, did he put all of the rings together and erase half of the universe and that was his response.