Well, it's not like Call of Gamma-Cthulhu would have to be serious. The arrival of the Strange Aeon could be an alternate timeline that merged with Earth, probably along the north-eastern coast of the United States (now known as the Gread Old Coast). Fighting crazy cultists and freaky fish guys would fit right in with the rest of the insanity that is Gamma World. Especially if the antagonists have really ridiculous and impractical ambitions such as world domination. Cthulhu's roles could range from the ultimate big bad to a passive, benign figure that imparts boons to the players via random mutations or mind-bending dreams.