Gue'vesa are human auxiliaries serving in the Tau Empire.
See, the Space Communists are a bit odd in that they aren't alone in their cause; unlike every other faction in the 41st millennium, the Tau are willing to allow non-Tau to serve the empire. Other races abound in the Tau empire, including Kroot and Vespids, but the most popular (at least on /tg/) are humans that joined the Empire. Humans working for the Tau empire are called "Gue'vesa," which quite literally means "human helpers" in the Tau language; ironically, the Imperium's grunts joke it means "Human Cannon Fodder", despite the fact the Gue'vesa are treated much better than themselves.
Most of the Gue'vesa come from Imperial Guard units abandoned in Tau space at the end of the Damocles Crusade, though they are mostly garrisoned on their own worlds and rarely participate in Tau crusades. Gue'vesa in conquering armies of Tau Empire are often the traitor guard and/or PDF of the planet they currently attack, converted by propaganda (often for decades before actual attack) or (according to Imperial Narrators) captured and brainwashed.
How This Happened
The Tau had finally gone and pissed the Imperium of Man off but good, as they had established a series of trade agreements with Imperial Worlds on the frontier, near the Damocles Gulf - and had begun trading goods and technology with the humans of the frontier. Naturally, since the Imperium immediately refers to any contact with Xenos that doesn't involve killing them with fire as Heresy, the Administratum flew into a rage that would make an Angry Marine titter with schoolgirlish glee were they capable of such. After fucking around for a while they eventually got a hugeass fleet into the sector about a century later and began blowing the fuck out of everything and pushing deep into Tau Empire territory.
The Imperial rapetrain ground to an abrupt halt when they came across the Dal'yth sept, which basically decided to do a bang-up impression of the Imperial Guard and hold the fucking line. Simultaneously amazed and horrified by the prospect any race, let alone a bunch of blue-skinned unguligrade alien space communists, could have the mighty balls to stand up to the Imperium in a stand-up drag-down fight, the Imperium was momentarily given pause - before it resumed doing its usual thing and blowing the fuck out of its enemies. Turns out the Tau had been deliberately giving ground to the Imperial crusaders, evacuating what they could and sacrificing several worlds to buy the Empire enough time to recall their forces elsewhere to make a stand.
Sadly for the Imperium of Man, the Tau did not relent, and showed a balls-out stubbornness that would make any Commissar shed manly tears of pride and RAGE. Even still, with the Imperials having a mighty hard time gaining ground on the Tau - the space communists had superior firepower and were fighting defensively in their home territory, which was a fight that was not very much to the Imperials' advantage, and what originally was expected to be a fairly short affair wound up embroiling the entire sector in a lengthy, drawn-out conflict. The Imperium was winning, slowly but surely, but at the rate they were progressing, it would be an eternity and a day before they could claim to have won the war - and likely trillions more lives. Moreover, though the Imperium had a gigantic manpower advantage (one that got bigger as the war dragged on), the war basically stalemated (Imperial Commanders blame Fish of Fury for this), with gigantic losses on both sides wearing down both the Imperials and Tau.
After it became painfully apparent that both sides were getting absolutely nowhere this way, the Imperium eventually relented and entered into meetings with the Tau Empire for cease-fire talks. Whilst neither side trusted the other one bit (this is the universe of grimdark after all), the meetings were basically successful, in no small part due to the then-new threat of Hive Fleet Behemoth moving in to rape Ultramar. This forced the Imperium to stop trolling the Tau diplomats and agree to the fucking cease-fire in order to rush to Macragge before the Hive Fleet could get there. This represented the end of the Damocles Crusade, and the beginning of a drawn out cold war between the Tau and the Imperium, where fighting is restricted to small skirmishes and brush fire conflicts that both sides try to avoid escalating (the Imperium because they have more important shit to worry about, the Tau because they do not want to get bumped to the top of the Imperium's threat list in the Ultima Segmentum and get hit with another crusade.)
This sudden redeployment left a shitload of Imperial personnel stranded, especially those that had been captured by the Tau, and a lot of humans remained in formerly Imperial territory that was now well within Tau space. Most of these humans were offered an agreement by the Tau to join the Tau Empire, and a great many did so, as the choice was between the POW camp or becoming Tau Empire Citizens. Like deciding between fine wine and a toilet full of spoiled meat, this was not exactly a difficult fucking choice.
Those humans who have joined the Tau empire have been provided with the technology required to prevail on the Tau frontier. They have access to their own production capabilities, allowing them to construct equipment ranging from farming and mining tools to Flashlights. Gue'vesa actively working for the Tau are also capable of using Pulse weapons, whilst Gue'vesa assigned to planetary defense duty generally lack these - but have access to your usual stock of fuck-your-shit-up ordnance.
Since some of these armies are self-contained the Tau have developed terms to define the ranks of the humans, for Gue'vesa soldiers is "Gue'vesa'la," and the Tau term for their Sergeants is "Gue'vesa'ui." Similarly, high-ranking ones are "Gue'vesa'vre" and Commanders are "Gue'vesa'el." High-level Gue'vesa commanders (ones comparable to a planetary governor) are called "Gue'vesa'o."
Gue'vesa and the Greater Good
Depending on the source you may have very different perspectives about what is for a human to be a citizen of the Tau Empire, Imperial authorities will insist the turncoats are either sterilized, used as cannon fodder and/or live in the worst possible conditions, with stories about the Tau replacing human population from conquered worlds with their own race, indeed a grain of salt maybe required when listening to a faction which enslaves, lobotomizes, burns alive and uses as cannon fodder their own, not that this bothers the high echelons really much, neither you citizen.
On the other hand, tau centered material and even a few novels of Black Library show the integrated humans as having better life standards and liberties than those from the Imperium, with enlightened views such as the Gue'vesa being allowed to worship the Emperor (sans the most fanatical aspects), their planets receiving extensive terraforming treatment to heal the damage done by imperial industry and being allowed to access education, health and fair work treatment. It's important to notice this may actually depend on the level of stability of the planet, as tau planets in state of warfare may put restrictions and even suspicion upon the Gue'vesa and confidence may demand to be earned.
All in all we may safely conclude that while life may still be harsh for a Gue'vesa colonist and restrictions of rank within the empire hierarchy may be applied, being a human in the Tau dominions is still largely preferable to living in most imperial planets, and the Gue'vesa may show as much conviction in the Greater Good as other members of the Tau Empire (and it's a no brainer to know what their fates may be if the Imperium ever defeated the Tau).
The Gue'vesa in crunch
The Gue'vesa are basically Imperial Guardsmen working for Tau. There are two versions of them - auxiliary troopers working for the Tau Empire (which can be fielded in a Tau army), or actual Imperial Guard regiments (made using the Guard's codex) that are just modified to look a bit more Tau-flavored and are used to represent Gue'vesa troops serving as colonial Planetary Defense Forces (for example). The Tau have generally kept a pretty good relationship with the humans in their territory (the Tau scenario in Dark Crusade has them sterilizing rebellious humans, but this is non-canon to all true fatguys, though fun to make fun of, and given the setting, sterilization's getting off light.), and the biggest fears that most humans in Tau space have is the Imperium getting pissed enough to come back and try to finish the fucking job.
AS previously pointed there two options for fielding a Gue'vesa army; those maintaining self-contained armies on behalf of the Tau (those using the Imperial Guard codex), and those using human auxiliaries to Tau forces (via chapter approved rules that are now of questionable tournament legality). It is also possible to represent them by using the Tau Codex and simply replacing the Tau models with human variants, and this is legal, but it will result in bricks being shat by other Tau players. As of 6th edition, you can also simply add some Imperial Guardsmen under the Allies rules; however if you're concerned with the fluff (and if you're trying to play Gue'vesa it's a safe bet to say you are), it's a little inconvenient that IG are Allies of Convenience to Tau, so they won't entirely function as a single army.
Gue'vesa auxiliaries working for the Tau are an unusual unit; mostly armed with Lasguns, they provide Tau with a combination of numbers and slightly better melee capability. One nice thing is that unlike the Kroot, the Gue'vesa aren't morons, and thus can equip specialized Tau-manufactured equipment, such as Markerlights, EMP Grenades, and pulse weapons. The fact that you get pretty good numbers per squad and the fact that they're kinda inexpensive makes them a pretty nice thing to have for a Tau army low on its numbers. Their one drawback is that they automatically are favored enemy for Imperial troops, who will hit them automatically in close-combat on a 3 or better.
In spite of this, Gue'vesa make a damned fine Tarpit unit. They are shooty enough to actually cause some damage to squads that aren't ready for them, their Gue'vesa'la can drop a Markerlight, and they cost notably less than even Kroot Carnivores. They also have an armor save (however shitty). They cannot infiltrate or use transports (which sucks), but their sheer cost and mass makes them a damned fine meat-shield unit for those who aren't terribly fond of using blobs of Kroot. Gue'vesa that do well enough are frequently given better equipment and promoted, with veteran Gue'vesa supposedly being eventually allowed to join Fire Warrior teams.
Gue'vesa armies made using the Guard codex are Imperial Guard units in every way but name and appearance.
This is basically also true of those playing Gue'vesa using the Tau codex.
Private Jeakim Slovaz was formerly an Imperial Guardsman who served in the 19th Brimlock Dragoons. Apparently, the regiment was abandoned in Tau space following the end of the Damocles Gulf Crusade (which was a prolonged engagement between the Imperium of Man and the Tau Empire). Because all of the their Commissars had (supposedly) been *blammed* by the Tau, the regiment decided to surrender before they all got fucked up as well. The only details following this incident came from a Tau propaganda campaign (not necessarily a bad thing) involving Jeakim Slovaz who told of the humane treatment that human prisoners of war could expect to receive from the Tau Empire
HERESY!. Understandably, the Imperium were not best pleased with Jeakim's involvement in the Tau propaganda campaign, and proceeded to label him has an EXCOMMUNICATE TRAITORIS due to his heretical interaction with foul xenos.
The Imperial Infantryman's Uplifting Primer on the subject. Tau propaganda looks much the same but with the roles reversed; fa/tg/uys joke that the Tau version is more accurate.
|Forces of the Tau|
|Command:||Cadre Fireblade - Ethereal - Tau Commander|
|Troops:||Drone Squadron - Fire Warrior Team - Pathfinder Team - Stealthsuit Team|
|Auxiliaries:|| Great Knarloc - Gue'vesa - Kroot Carnivore Squad - Kroot Hound |
Krootox - Lesser Knarloc - Nicassar - Vespid Stingwing
|Battlesuits:|| Battlesuits (XV02 Pilot Battlesuit - XV15 Stealthsuit - XV22 Command Suit |
XV25 Stealthsuit - XV46 Vanguard Void Suit - XV-8 Crisis Battlesuit
XV88 Broadside Battlesuit - XV9 Hazard Battlesuit - XV95 Ghostkeel Battlesuit
XV104 Riptide Battlesuit - XV107 R'varna Battlesuit - XV109 Y'Vahra Battlesuit
KV128 Stormsurge Ballistic Suit - KX139 Ta'Unar Supremacy Armour)
|Vehicles:||Devilfish - Hammerhead - Piranha - Sky Ray - Tetra|
|Flyers:||Barracuda - Manta - Orca - Razorshark - Remora - Sun Shark - Tiger Shark|