There's an OGL product out there called "Delta World" that looks to be somewhat like 5e Gamma World, except also with elves and dwarves and tieflings and things. I've been thinking about buying it.
I was running an Alternity version, but it died from lack of interest.
Of all the versions of GW over the years, the Alternity version might have been the worst. Though I do recall the one that had no index or table of contents and a randomly scrambled rule book.
Just here briefly to plug three post-apocalyptic games based on The Black Hack: The Rad Hack (the most Gamma World of these three), Wasted (a bit more infusion from Mad Max), and Barbarians of the Ruined Earth (full-on Thundarr the Barbarian). Good stuff!
That cover wasn't 1st edition. 3rd I believe. I owned that edition, but never played it (other than making 100 characters). But I wanted to post the cover because it is AWESOME.
The one that Tim posted was third edition. The one that witchdoctor posted was first edition (and my personal favorite). I've played every edition (including Amazing Engine), but you always remember your first.
Of course, none of this gets us any closer to playing. I would volunteer, but I already flaked on one game as GM. I need to get my s$%t in order before I try running something again.