It never made a lot of sense to me for AD&D/Gamma World crossover games to take place in the post-apocalytic North America that was the setting for the Gamma World game. Not that it particularly doesn't make sense; after all, a magical portal could just as easily transport folks across space as well as time and probability. But there would seem to be a certain symmetry in putting a Gamma World crossover in the Flanaess of the incredibly far future.
We know from the introduction to the 1983 boxed set that magic in Oerth was gradually receding in power and ubiquitousness. It doesn't seem too far a stretch to aim at a low magic setting (much more low than the current "main" campaign, and perhaps a "reborn art" that was lost in the "civilized" times leading up to the holocaust, and only now being rediscovered). The setting could be anywhere from 1,500 to 2,000 years in the future, after the standard apocalyptic war with nuclear/ biological/ chemical weapons. The basic geography would be the same; Rauxes and Greyhawk are just as likely to have been major metropolises that far in the future as Rome or London (albeit unrecognizable to their ancient inhabitants, of course). The intervening history and politics would be largely irrelevant, rendered moot by the complete fall of civilization.
The sentient races of humans, demi-humans, and humanoids could all be represented (both in pure form and as mutants-- mutant halflings!!!), as well as the "new" sentient races born of the bombs. The number of monsters would be impressive, as one could apply random mutation tables to the venerable beasties in the Monster Manuals (many, of course, having been driven to or near extinction as the world became more "civilized", and perhaps thriving again after the collapse). Extra-planar creatures would likely be found rarely if at all, industrial societies being not too palatible for their tastes, but perhaps rediscovering Oerth now that it's much less "civilized". Secret societies? I'm thinking 2,000 year old Scarlet Brotherhood.
The play would still have the usual high-tech gadgetry; powered armor, lasers, androids and robots, etc. The focus would be on the PC's quest to get back "home" (to the past, in their case). I think I'd aim for a Thundarr The Barbarian feel to this; "sorcery and super-science", with high-tech wizard/cyborg/mutants. Of course, having Celene split in two might just be a little too obvious. I've always thought Gamma World was a perfect setting to interject all sorts of pop culture references into the ruins; Bart Simpson dolls, unopened boxes of Twinkies that survived the blasts, etc. I think that would still work here, but since Oerth is a sort of parallel world to our own, the pop culture references would be slightly skewed. A movie poster for "Planet of the Orcs" found in an abandoned subway station, perhaps.
And, of course, nothing says that the future inhabitants of Oerth couldn't have launched their own interstellar colony ships. And that one suffered a chance encounter with a destructive radiation field...
I'm liking this idea!