This faction orginiated in an alternate Adventverse, in which clones were maltreated and openly hated, their punishments being much more sever for them then for anyone else. So a set of Officers namely: Gree, Cody, Rex, Appo, Bly, Neyo, Fordo, Bacara, Jet, Ponds, Daviss, Blackout, Trauma, Keller, Monnk, Stone, Keeli, Thire. Started a rebellion during which they severly crippled the CI, but at the point when their greatest victory was within their grasp, a riftstorm swallowed them, Kamino and Kuat. That was what brought them into the prime verse. There they immediatly declared war at the UNSC and attacked it, as they still thought them to be part of the CI, only after some skirmishes against the Phantom Legion, both sides realized what was realing going on. Then the fighting stopped and the GCA, became a neutral party.