Gamma Domes are fields made of pure gamma energy produced by Gamma Generators.
Once the dome expands, anyone who got caught inside will be turned into Gamma Monsters unless they wear protective radiation armor, given a special antidote or being an immortal.
It appeared in "Gamma World Pts. 1 & 2", when the Gamma Generator produces a dome of gamma energy turning everyone including the Avengers into Gamma Monsters who got caught in its expansion except for Thor and Samson who were captured. Hawkeye even went to Bruce Banner to give him an anti-radiation antidote to temporarily protect him from the dome before proceeding on his quest to save the Avengers and the world with Hulk.
After the Leader's defeat, Hulk had uprooted the Generator causing the dome to dissapate and then threw it into the far reaches of space where no one will use it for evil purposes. The dispersal of the Cube have reverted everyone back to normal.