Over on Greyhawk Grognard there was a thought-provoking post about Gamma World.
Why no nations in Gamma World?
It's a good question, although one James Ward answered in The Dragon #25. Cryptic Alliances are the nations. I always felt that way myself - you'd have villages, small towns, the occasional "free city" (maybe 2-3 of those across the whole campaign area). But no big states outside of the Cryptic Alliances.
In a way, modern groups like ISIS/ISIL are a pattern for a Cryptic Alliance. They're a Belief State. The founding principle of the state is a shared ideological view of the world - something that exactly describes groups like the Seekers or the Red Death or the Knights of Genetic Purity. They fill their ranks with true believers and exploit those in their area they can. In areas where the other power structures are stronger than the C.A., they act as small anti-establishment groups working to destabilize the area and gain control or to gain recruits and power for the base area.
I'd call these Belief States, since they aren't Nation-States (no "nation" or shared culture per se) but do share a world outlook.
Cryptic Alliances Acting As States
I think the C.A.s make good states. The big, land-grabby types (Ranks of the Fit, the Zoopremists, the Created, etc.) would have grabbed land. They'd also send out wild-eyed believers to new lands to gather recruits to bring back, to found new lands, or to loot for valuable artifacts.
Some of these states would work in a typical direct land-grabbing fashion. They'd expand their overall borders, pushing out as they had more ability to absorb neighboring populations.
Others would just be nomadic groups, and move as a large group with many affiliated small groups that splinter off or get sent off on their own.
Some of these states would go off of the "ink spot" theory. That is, send out a group to a likely place for a state. With all of the radioactive badlands, scorched lands, areas overgrown with nasty hostile vegetation, still-dangerous military fortifications operating on "kill anything that approaches" mode, wandering warbots, etc. you can't just expand quickly and evenly. You need recruits, you need weapons, you need the ability to colonize or extend supplies across the badlands to rich liveable lands. But you can send out colonies to especially rich or valuable spots and see if they can't make it on their own.
You would still get some non-C.A. states but they would be small and easily taken over by the larger, more organized, and more belief-centered alliances. Hard to run a tiny little democratic state when the Red Death rolls through or a large group of Zoopremists show up and tell you how its going to be.
You'd also get traders willing to go between the alliances and the non-allied (possibly tributary) areas. Some would be C.A. members (openly or secretly), some independents, some part of the minor power structures that are villages and towns.
You'd also get C.A. members all over the place - the self-declared allies, the deliberately sent out colonists, the looters, the recruiters. And the non-landed groups would do the same because they don't have a land to stay in.
Why Cryptic Alliances as States, and not just States?
Part of this is just Gamma World - the basis of the end of the world is a series of belief states fighting each other. The idea that the rise of the next is a warped and irradiated version of the same fits the apocalyptic feel of the setting. The Created and the Ranks of the Fit are not so far from the League of Free Men and the others who started the whole shebang.
You could relegate most Cryptic Alliances to just secret societies that exist within existing secular states. But I think it's a lot more fun to have the lands that belong to the Ranks of the Fit than to have the Kingdom of Loosyanna with some scheming sub-groups in it. It makes the land feel more threatening if there are only a limited amount of places you can go if you aren't part of some group. And it makes either joining a group - or founding your own - a bigger challenge. Most of the Cryptic Alliances just don't accept that other people's beliefs are valid too. That would also make the few free cities worth the name valuable prizes for all sides, dangerous pits of trouble, really interesting places to visit, etc. Being the exception (non-C.A. allied large power structure) not the rule (C.A. controlled) makes them really interesting. Like The Free City of Krakow in T2K.
And that's how I'd run Gamma World.