Have a look at
Apocalypse World by D. Vincent Baker. It's a good system built to run Apocalyptic games. But even if you have no interest in the system then it has great advise on how to capture the feel of post apocalypse in games.
EDIT: About running it in GURPS. GURPS is a great system for games that are built as small unit tactical missions. Or overtly political games. I'm not sure it's a good system for Resource Management or running a town. But having said that I can't think of a system that has that built in. I think even Apocalypse world mentioned above just has a "things are getting worse" meter but no hard numbers.