Future Gohan lost his left arm during a battle against the Future Androids 17 and 18 in his timeline. Specifically, he lost his arm while protecting Future Trunks from a deadly ki blast bombardment unleashed by the two androids. The intense energy attacks caused his left arm to be completely blown off as he shielded Trunks, who was unconscious at the time. Despite having a Senzu Bean that could heal him, Future Gohan gave it to Trunks instead, sacrificing his own recovery to ensure Trunks' survival. This event is depicted in the Dragon Ball Z: The History of Trunks special and related media, though the exact details vary slightly between the anime and manga continuities. In the anime, the ki blast bombardment by the androids is the direct cause of the arm loss