The Sacrifice of Gamora was the ultimate step that was required in order for Thanos to obtain the Soul Stone. When the Red Skull informed Thanos that he must sacrifice something that he loved to obtain the Soul Stone, he threw his adopted daughter off the cliff with great sadness.
Gamora was tasked by the Thanos to locate and retrieve the Soul Stone, which Thanos needed to complete his Infinity Gauntlet and fulfill his project of exterminating half of all life in the universe. Gamora successfully retrieved a map leading to the Soul Stone on Vormir. However, Gamora chose not to reveal her achievement to Thanos, pretending that she had failed and burned the map.
However, Thanos was not fooled by Gamora's lies and knew that she had found the location of the Soul Stone. Sometime after Ego's Expansion, Nebula infiltrated the Sanctuary II, and attempted to assassinate Thanos, only to be thwarted and kept in custody. Probing Nebula's cybernetic memory banks, Thanos' suspicions were confirmed, that Gamora knew of the Soul Stone's location and had secretly admitted it to Nebula.
Having decided to move further in his quest, Thanos lured the Guardians of the Galaxy to Knowhere where he took the Reality Stone, and ambushed them. Before Star-Lord could kill Gamora to prevent the knowledge of the Soul Stone falling into his hands, Thanos captured Gamora, taking her back to the Sanctuary II.
Despite Gamora's attempts to lie, Thanos revealed that he knew Gamora had found the Soul Stone, through the memories of the captive Nebula, and began to brutally torture her to force Gamora to reveal its location. Unable to let her sister die, Gamora eventually caved in, and revealed the location of the Soul Stone to Thanos, on Vormir. To confirm this, Thanos and Gamora left for Vormir, with him threatening to kill Nebula if she was found to be lying.
Upon arriving on Vormir, Thanos, and Gamora were welcomed by Red Skull who was transported to Vormir by the Tesseract to become the keeper of the Soul Stone. When Thanos questioned him as to how he knew their identities, the Stonekeeper informed them that he was cursed to know all who journeyed to the Soul Stone's altar. However, when Thanos demanded the whereabouts of the Stone, the Stonekeeper cautioned him that the Stone itself comes at a terrible price that none are prepared to give.
As Red Skull led Thanos and Gamora deeper into the altar, he explained how he came to become the Soul Stone's guardian a lifetime ago on Earth, much like Thanos, he too originally sought the Infinity Stones, even once wielding the Space Stone in his hand. However, the Stone judged him unworthy of its power and banished him to Vormir to become the eternal Stonekeeper. As he concluded his story, he finally brought Thanos and Gamora to a sacrificial cliff at the end of the altar.
Red Skull informed them that in order to obtain the Stone, one would have to sacrifice the one being they love most to the cliff, astonishing Thanos. This revelation greatly amused Gamora, who taunted her adoptive father, believing that he loved nothing whatsoever, and this was the universe's way of ironically punishing him for his bloody quest of destruction. Silently, Thanos turned around, revealing that he was crying. Believing he was weeping due to his failure, Gamora continued to taunt Thanos.
However, Red Skull saw his true intentions, informing Gamora that Thanos was, in fact, weeping for Gamora, as he truly loved her as a daughter, and was heartbroken that he had to sacrifice her to get the Soul Stone. Disbelieving, Gamora could not comprehend that Thanos ever showed genuine love for her. However, upon realizing it was true, Gamora quickly took her switchblade from Thanos' belt. Drawing it, she attempted to kill herself, only to be fooled by the Reality Stone, opening her hand to reveal only bubbles.
Tearfully, Thanos apologized to Gamora for what he was about to do, before grabbing her arm and dragging her screaming, towards the edge of the cliff. Despite his reluctance and her frantic screams, Thanos threw Gamora off the cliff to her death. As she died, the altar began to release energy, consuming Thanos as he tearfully shut his eyes. He eventually woke up, finding himself lying in a shallow pool at the base of the mountain, with the Soul Stone gleaming in his hand.
Thanos, now equipped with four Infinity Stones, would leave for Titan to rendezvous with his children, where he would instead be ambushed by the Avengers, the Guardians of the Galaxy and Stephen Strange. The heroes were able to subdue Thanos, with Mantis using her powers to calm his fury. The heroes were about to remove the Infinity Gauntlet until Star-Lord began to ask about Gamora. When Mantis stated that Thanos was mourning, something which shocked all of them, Nebula realized in horror that Thanos had murdered Gamora to obtain the Soul Stone. Furious at this revelation, Star-Lord lashed out at Thanos, hitting him numerous times. Thanos was able to break the trance Mantis had him in, recover his gauntlet and attack his would-be captors. The reminder of his sacrifice of Gamora served to make him more determined than ever to complete his goal so as to not let it be in vain.
Eventually, Thanos was able to collect all six Infinity Stones. He snapped his fingers and wiped out half of the universe's population. Thanos found himself inside the world of the Soul Stone, where he met a reflection of a young Gamora, telling her he finally completed his mission. When she asked what it cost, he solemnly replied, "Everything," as he had lost his beloved daughter in the process. Gamora sadly smiled before Thanos returned to reality.
Five years later, while the Avengers traveled through the past to recover the Infinity Stones before Thanos destroyed them in 2018, Hawkeye and Black Widow went to Vormir and met Red Skull, who informed them about the love sacrifice. Although Hawkeye initially dismissed the Red Skull's warning as a lie, Black Widow made him realize it was true as Thanos originally arrived on Vormir accompanied by Gamora but she didn't return with him.
Though they fought against each other for who had to die, Black Widow ended up rescuing Hawkeye from killing himself and let herself to fall to her death, killing herself and enabling Hawkeye to take the Soul Stone as well freeing the Red Skull from his curse, and thus creating a revised timeline in which Gamora's sacrifice never came to happen.