A large number of gue'vesa can trace their origins back to the Damocles Gulf Crusade. When the war against the Tau ground to a stalemate, the Imperium decided to retreat and redeploy the forces against other threats, such as Hive Fleet Behemoth. The withdrawal was so fast, however, that many Imperial Guardsmen were simply abandoned and stranded in Tau-controlled regions of space. O'Shovah, or Commander Farsight as he is known in the Imperium gave these humans the option of joining the Tau Empire, an offer that most accepted. They were treated well by the Tau, and were even allowed to continue worshipping the God Emperor. (Minus all the fanaticism and xenophobia, of course.) As time passed, the guardsmen and their children turned from reluctant converts to willing followers of the Greater Good.
Not all Gue'vesa have that background, though. The Tau regularly take Imperial Guard prisoners and try to convert them. Others actually support the Tau to begin with. In the Ciaphas Cain novel, "For The Emperor", there are a large number of people who can be considered Gue'vesa, as they support the Tau Empire over the Imperium, even though the world in question is still under the jurisdiction of the Imperium. A notable example of this occurred on Taros. When it was annexed by the Tau Empire, not only did the population not oppose the move, but the PDF actually fought against the Imperial forces sent to reclaim the world.
Regardless of origin, the Imperium views these men as traitors who need to die.
Gue'vesa are now a common sight in the Tau Empire, especially in the western portions. In general, they are content and treated well under Tau rule, but still often worship the Emperor in a moderate manner. These humans maintain colonies and are given technology to survive on the frontier. They have some manufacturing capabilities and can produce things ranging from farming tools to copies of the lasgun, which they use to defend themselves on the frontier.
In times of war, the Tau give them weaponry such as pulse rifles, pulse carbines, and more specialized equipment and have them serve in the Auxiliary. Because of there , these auxiliaries have become a respected part of the Tau military in the border regions, where they fill a tactical niche between the barbaric Kroot and the mainstay of Tau armies, the Fire Warrior teams.