As Thanos, the Black Order, and his army invaded Zen-Whoberi as part of his mission to balance half of all living forms in the universe, Gamora's mother witnessed her people being murdered by the intruders. She attempted to hide in her domicile with her young daughter, Gamora, only for a Chitauri warrior to discover the two. Gamora's mother was executed by the Chitauri, yet Gamora was spared by the Mad Titan, who adopted her and make her one of his assassins.
"He's not my father. When Thanos took my home world, he killed my parents in front of me. He tortured me, turned me into a weapon. When he said he was going to destroy an entire planet for Ronan, I...I couldn't stand by and..."
In 2014, during the quest for the Power Stone on Knowhere, Gamora took the opportunity to tell Star-Lord about the fate of both her mother and father, telling him that Thanos got them killed, assuring him that the reason behind her motives to save Xandar was that she didn't want Thanos nor Ronan the Accuser to destroy an entire planet again and avoid others to suffer what she did.