A bit off topic here, but why are there still 2 magazines? With DDI, you get a subscription to both, so why keep them apart?
My guess is two reasons: 1. Nostalgia, 2. Perhaps makes it easier to set up two different (though both headed by Steve Winter) for production -- one focusing on DM material, the other on player material.
As to the original question, I would be surprised if there wasn't some support given the amount of hype that Gamma World has been getting from WotC. As I understand it (from talking to the owner of my FLGS), WotC is expecting Gamma World to be as succesful as a fully self-contained RPG, not just as succesful as say Martial Power 2. At the very least, I would expect an adventure or two published in Dungeon.
As for supporting setting-specific material, yes, there will always be people who complain when even one article in an issue isn't "usuable" by them. The problem is though, there isn't any way to avoid this. For instance, I'm not overly enamored with the warlord class (just my opinion). As a result, I can't envision myself ever playing a warlord. That doesn't mean though that WotC shouldn't publish warlord articles though, even though that material isn't "usable" by me.
Obviously, setting specific material is a different issue altogether. Clearly, WotC is trying to make all settings generic (hence Dragonmarks usuable in FR, etc.), but I still don't see a problem with some setting specific support. In fact, I think it would be a good use of space to devote one or two articles a month to a particular setting, just not the same one. So, November sees an article on Eberron, December gets one on Dark Sun and one on FR, then January gets a Gamma World article, etc. This way, everyone gets a little taste without an entire issue being rendered "useless".