In the original version of Gamma World, Gamma Earth was created by a mysterious secret society of high-tech terrorists known as the Apocolypse in the year 2310. The world had reached a golden age of prosperity and in boredom had turned to political experimentation, resulting in the creation of increasingly fanatical revolutionary activist groups known as Cryptic Alliances, which began to conduct covert wars of assassination and terror amongst one another. The Apocalypse stepped forward at the end of this era to attempt to end the violence with an ultimatum promising mutually assured destruction should they continue on their current course. Ignored, they unleased a mass bombing which vaporised every national capital. The enraged survivors launched a massive manhunt for those responsible and in a final standoff against the armies of the entire planet unleashed their unthinkable final solution--a genocidal counterattack that boiled oceans and buckled techtonic plates and eliminated every society on Earth. The current era is 2471, 161 years later in a time called the Black Years. Mutation is rampant, evolving people into monsters and plants and animals into people.
In the fifth edition of Gamma World, people in the near future finally fix the world's problems when a race of aliens, who had been observing Earth for some time, decide we are ready and establish contact. The relationship ends up being mutually beneficial as each race has developed technologies the other race is in awe of, and together the two races are able to achieve a golden age of accomplishment. A small group of humans harbor suspicions that the aliens are too good to be true and seek to goad to antagonize them into revealing their "true colors". In one such attack, a group of humans is able to launch a sneak attack inside the alien mothership. Automated defenses carry out a drone controlled counterattack against Earth. Humans unaware of the instigating attack throw everything they have against the aliens, destroying their mothership and devastating much of the planet. Generations pass and humans, aliens and the robots and uplifted sentient humanoid plants and animals they once created share a planet scoured by the weird mutagenic radiation of the alien weapons as well as the more traditionally toxic and deadly nuclear radiation of our own. Old hatreds have simmered down into lingering resentment, the decendants of the stranded aliens have integrated as best they can. Sasquatches forced from their quiet forest dwelling existance have adopted a warrior raider culture.
In the sixth incarnation of Gamma World, technological progress continues to advance to a point where artificial intelligence and the invention of new sentient races become ordinary human activities. People can be raised from the dead or reborn as machine consciousnesses or in any form of body they wish. Such perverse and disrespectful behavior eventually leads to vast injustice and hubris. Inevitably the inequalities of this world lead to war as servitor species and enslaved artificial intelligences turn on their masters and various high-tech factions seek to remold the world in their image. When it's all over, mad science and devastation mix in a world ruled by mutation, nanotechnology, and horror.
The seventh edition version of Gamma World differs greatly from it's predecessors in that the catastrophe that caused it was scientific rather than military. Scientists working at the CERN facility in Switzerland were conducting superconductor research on the nature and composition of reality, when something went horribly wrong. The disasterous explosion that resulted, known to future generations as the Big Mistake, tore apart the boundaries between alternate univeses. Every reality poured into one another until all that remained was a fused crazy quilt of alternate histories, dinosaur worlds and robot empires. This new reality became known as Gamma Terra. Most societies just fell apart in the craziness, but three were monolithic enough that they persisted: The Empire of Ishtar, The Machine Civilization of Xi, and the Thu Marth shadow government run from an installation known as Area 52. These nations met, tried to deal with each other, but relations quickly degenerated into open warfare. The ruins of this conflict, 150 years after the Big Mistake, are the setting for the game, in a devastated merged world dotted with relics of Omega Technology from these three extinct civilizations.