Gamma Terra is the setting in the long-running role-playing game Gamma World. In earlier editions it was referred to as Gamma World.
"Gamma World takes place in the mid-25th century, more than a century after a second nuclear war had decimated human civilization. The game's designers took inspiration from the post-apocalyptic novels and movies of the 1950s, 1960s, and 1970s; the first edition rules cite Brian Aldiss's Hothouse, Andre Norton's Star Man's Son (also published as Daybreak - 2250 A.D.), Sterling E. Lanier's Hiero's Journey, and Ralph Bakshi's Wizards.
The war that destroyed civilization in Gamma World is only vaguely described in most editions of the game, and what details are provided change from version to version. The first two editions place a first nuclear war near the end of the 21st century, with the final war in the years AD 2309-2322, and ascribe the final annihilation to a terrorist group called "The Apocalypse" and the ensuing retaliation by surviving factions. The 2010 edition introduces a significantly different backstory that attributes the destruction of civilization to the activation of the Large Hadron Collider, which caused multiple realities to exchange features in an event known as "The Big Mistake". All editions, however, agree that the cataclysm destroyed all government and society beyond a village scale, plunging the world into a Dark Age. In many editions of the game technology is at best quasi-medieval (in the first edition, the crossbow is described as "the ultimate weapon" for most Gamma World societies). Some, such as the 2010 edition, feature advanced, but rare, technology that is risky to use due to the average Gamma World character not knowing how to properly operate such devices. The post-apocalyptic inhabitants of Earth now refer to their planet as "Gamma World" (or "Gamma Terra" in later editions).
Gamma World is a chaotic, dangerous environment that little resembles pre-apocalyptic Earth. The weapons unleashed during the final war were strong enough to alter coastlines, level cities, and leave large areas of land lethally radioactive. These future weapons bathed the surviving life of Earth in unspecified forms of radiation and biochemical agents, producing widespread, permanent mutations among humans, animals, and plants. As a result, fantastic mutations such as multiple limbs, super strength, and psychic powers are relatively common. (Random tables of such improbable mutations are a hallmark of every edition of Gamma World.) Many animals and plants are sentient, semi-civilized species competing with surviving humans. Both humans and non-humans have lost most knowledge of the pre-war humans, whom Gamma World's inhabitants refer to as "the Ancients". The only group with significant knowledge of the Ancients are isolated robots and other artificial intelligences that survived the war—though these machines tend to be damaged, in ill-repair, or insanely hostile to organic beings.
Gamma World player characters include unmutated humans (referred to as "Pure Strain Humans" in most editions), mutated humans, sentient animals or plants, and androids. Characters explore Ancient ruins and strange post-apocalyptic societies to gain knowledge of the Ancients and social status for themselves. Common adventure themes involve protecting fragile post-apocalypse societies, retrieving Ancient "artifacts" (science fiction gadgetry such as power armor, laser pistols, and anti-grav sleds), or mere survival against the multifarious dangers of the future (such as gun-toting mutant rabbits, rampaging ancient death machines, or other Gamma Worlders bent on mayhem).
A recurrent source of conflict on Gamma World is the rivalry among the "Cryptic Alliances", semi-secret societies whose ideological agendas—usually verging on monomania—often bring them into conflict with the rest of the Gamma World. For example, the Pure Strain Human "Knights of Genetic Purity" seek to exterminate all mutants, while the all-mutant "Iron Society" wants to eliminate unmutated humans. Other rivalries involve attitudes towards Ancient technology, with some Alliances (such as "The Restorationists") seeking to rebuild Ancient society, while others (such as "The Seekers") want to destroy remaining artifacts."
Source: Wikipedia, "Gamma World", available under the CC-BY-SA License.