Visiting Gamma World
I found this all rather interesting, especially since I had no real idea about other RPGs at the time. Moreover, my access to other RPGs was limited as well, both by availability and cost.
The cost is easier to explain. As a child in middle school I just didn't have a ton of disposable income. Actually, pretty much all of my income was disposable, I just didn't have all that much of it. And those game books were expensive - as I recall, the Players Handbook ran for $12 and Dungeon Masters Guide for $20. As a DM I of course got the Dungeon Masters Guide first. A bit confusing when you tried to combine it with the D&D Basic and Expert rules but I was far from the only person who did that.
As far as availability goes. My initial source of gaming books were from the Toys R Us store in nearby Waterbury, Connecticut. Believe it or not, back in the early 80s one could get D&D and AD&D products pretty much everywhere. I know I bought Deities and Demigods and Expedition to Barrier Peaks from a Toys R Us. However, Toys R Us tended to focus just on D&D and AD&D along with, I recall, the TSR minigames. I was therefore quite pleases when a hobby store opened in my hometown of Naugatuck. It was a tiny shop called Witzend. But it had so much stuff. Lots of black books from a game called Traveller. Something I never got at the time as I could never find the main rules for the game. But it never had Gamma World despite me wanting it so bad, both from the mysterious description in the Dungeon Masters Guide as well as the mysterious sounding advertisement in TSR's "Gateway to Adventure" catalog.
I was home sick from school once when my mother offered to get me something from the hobby store. I listed some of the books I was looking for - imagine my surprise when she came home with Gamma World, not the 1st edition version from the catalog but the new 2nd edition (that probably explains why I couldn't find the darn game - Gamma World is notorious for going out of print and returning with a new edition).
I greatly enjoyed the game - and I will probably review it in full at some point in the future. Despite being a new edition, it certainly appeared compatible with the conversion rules in the Dungeon Masters Guide (I believe the 1st and 2nd editions of the game are the only two which maintained full compatibility with each other). The rules seemed familiar to the D&D rules I knew, though it was clearly a different game. I was also a little creeped out by it, growing up in the final surges of Cold War with nuclear armageddon around the corner.
Despite being a little frightened of the subject matter, Gamma World had something which made it feel more fun. Whatever apocalypse befell the Earth (and the game was vague on that), it was not an apocalypse inflicted on our Earth but rather one hundreds of years in the future, a world with blasters, lasers, portable nuclear power, broadcast power generators, etc. That made it feel a lot "safer" to pretend with.
My gaming group of course tried it out. We did what seemed natural - our AD&D characters paid a vist to Gamma World (by this time we'd figured out D&D and AD&D were two separate games - who at TSR thought that strategy was a good one?) I don't recall the reason I gave as DM for crossing worlds - it certainly wasn't for some epic reason. I think I had a MacGuffin where some item they were looking for was located on Gamma World. We used the Rite of Passage adventure included in the 2nd edition game. This adventure had the characters go on a manhood ritual for their tribe to visit the ruins of Pitz Burke (Pittsburgh) to retrieve an artifact and return.
I don't recall much of the adventure save it was a lot of fun. One memorable incident was when they tried to gain access to a city-state and the guards didn't like their looks, causing the magic-user to intimidate them with his fireballs, becoming regarded as a deity. Gamma World has a reputation for "gonzo" gaming, a bit of whackiness. I think it is a mistake to try to incorporate that in the rules, something I get an impression is being done with the latest incarnation of Gamma World with mutations you can gain and lose by trading mutation cards. Rather I think it was a natural outgrowth of play. Yes, you were going through the ruins of a lost civilization and facing all sorts of odd mutations. But to be honest, that greatly resembled most D&D games of the day.
With the Dungeon Masters Guide giving such "permission", I recall running and participating in a lot of games where visiting other worlds was commonplace. TSR and Dragon magazine certainly encouraged this, with adventures taking place on Earth (Dragon magazine's "City Beyond the Gate"), in Wonderland (the Dungeonland adventure), aboard a spaceship (Expedition to Barrier Peaks), etc. D&D featured Blackmoor which had science fiction elements to it. In Dragon magazine Gary Gygax discussed how his Greyhawk campaign featured visits to the Mars of Edgar Rice Burroughs.
This sort of world-hopping seems to have diminished in gaming of late which to my mind is a bit of a pity. It certainly seemed prevalent in the early days of the hobby and also was something found in much of the literature which was the inspiration for D&D.