- "The entire time I knew Thanos, he only ever had one goal. To bring balance to the universe by wiping out half of all life. He used to kill people planet by planet, massacre by massacre."
"Including my own."
"If he gets all six Infinity Stones... he can do it with the snap of his fingers like this."
- ―Gamora and Drax the Destroyer[src]
In accordance with his plan to bring stability to the universe, Thanos embarked on a campaign to conquer planets and destroy half of all life on them. Seeking henchmen and warriors who would serve his cause, Thanos adopted orphans from the planets he invaded, creating the "Black Order". The members included Ebony Maw, Proxima Midnight, Corvus Glaive, Cull Obsidian, Nebula, and Gamora.
Ultimately, Thanos learned of the Infinity Stones- six powerful objects that each represented an aspect of the universe- had the capability to annihilate half of all living creatures in the universe. In an effort to fulfill their father's plans, the Black Order were instructed to conduct massacres on different planets and hunt for the Infinity Stones. In once instance, Gamora came in the possession of a map that led her to the location of the Soul Stone, sharing this discovery only with Nebula, whom she trusted out of all siblings.
Under Thanos's orders, Gamora and Nebula invaded Korbin and massacred half of its population in accordance with his ideology of balancing the universe. Despite having killed a few soldiers, Nebula was caught off guard and was attacked by a soldier who threw a grenade in her direction. As the guard attempted to kill her, Gamora intervened and killed the soldier.
With the threat neutralized, Gamora extended her hand at Nebula, who insisted that she did not need her assistance in the fight. However, Gamora responded that Nebula always needed her help as in battle she would always fail during the mission and run into a predicament. Following this, Gamora told Nebula that Thanos found the Power Stone, prompting the sisters to return to their Q-Ship.
Gamora and Nebula reunited with Thanos aboard the Q-Ship, where he announced that he was going to dispatch them to the Dark Aster, the Kree warship that belonged to his associate, Ronan the Accuser, who had located the Orb on Morag, which contained the Power Stone. Despite Gamora's warnings that Ronan would be uncomfortable and annoyed with their presence, Thanos noted that his alternative was death and his obsession with the Kree-Nova War clouded his judgments.
As the two sisters were dispatched to the Dark Aster, Ronan already sent Korath the Pursuer to retrieve the Orb on Morag. However, his efforts were thwarted by the intervention Star-Lord, a former Ravager, who stole the Orb and planned to sell it to the Broker on Xandar. After Korath reported his unsuccessful mission to Ronan, Nebula, and Gamora, Ronan instructed Gamora to procure the Orb on Xandar, threatening her with a severe consequence if she failed. Unbeknownst to both Ronan and Thanos, Gamora harbored hatred for both of them and planned to sell the Orb to the Collector in an attempt to disappear from Thanos entirely. Her ulterior motives coupled with her fight with Star-Lord, Rocket Raccoon, and Groot on Xandar subsequently led to their arrest and incarceration at the Kyln.