Grand Admiral of the Fleet of the Spartoi Empire, Gareth is uncle to Emperor Jason of Spartax.
Trusted by his nephew, Gareth was intrusted with retrieving Jason's love Meredith Quill and his son Peter from the planet Earth. Instead of saving them, Gareth contacted the ruthless and powerful Kyras Shakati to have them killed. They would send the Ariguan Rruothk'ar to carry out the murders, while Gareth told his nephew they died in childbirth.
Gareth would later stage a coup d'état in order to assume power over the empire. To fund this, he authorized the subjugation of alien worlds and their destruction. Unknown to Gareth, Jason's son survived to become Starlord, who would eventually learn of Gareth's plans and face him in battle. Gareth was killed at the end of their fight.