Gamma World occupies a similar place in my heart as Boot Hill. It was a TSR rpg that I never owned as a kid, but a member of my gaming group had a copy. This was back when we were all young and stupid. I'm not sure we realized that just buying an RPG wasn't enough. Someone had to break down and run the mofo. Usually that someone was the owner of the game. Gamma World and Boot Hill also share a connection in that official conversion rules to and from AD&D were provided in the first edition Dungeon Masters Guide in the wonderfully named sections "Sixguns & Sorcery" and "Mutants & Magic". I've sometimes wondered if a hybrid game of all three ("Sixguns & Mutants?") would be feasible.
According to informants on RPG.net the first two editions are very similar rules-wise, the second edition merely provided clarifications and an intro scenario. The 2nd is what Shawn Watson owned back in the day as well. Five additional products appear to be relevant, modules GW1 through GW4 and the inevitable judges screen and mini-module.