Spartan-III Gamma Company was the last of three Spartan-III companies to be recruited, trained, and put into service. Their training began in 2545, after being recruited as children orphaned by the war. Gamma Company, like all Spartan-III companies, were trained on the planet Onyx, which lasted for six years until they received their augmentations at the end of their training. Gamma company stands out from the first two Spartan-III companies for training an additional year after their augmentations, and receiving additional mind-altering drugs during the augmentation process that allowed them to better survive in combat. The Spartan-III program was run by former Spartan-II Kurt-051, and Senior Chief Petty Officer Franklin Mendez. Spartan-IIIs were known for being a cheaper version of the Spartan-II program, which included wearing Semi-Powered Infiltration (SPI) armor instead of the MJOLNIR that Spartan-IIs wore.
These events are depicted in the novel Halo: Ghosts of Onyx.