This is a fun map I used on my old server and was used on a few other servers I played on. It is large with lots of interesting features. Starting from the top it has a medium sized fallout style vault, a large Blown up bunker, a large Military bunker, a small science bunker, a Gas Station with a secret tunnel to the military bunker, and finally a town.
The map is on addons menu and there is a night and prop fix version out there.
http://www.garrysmod.org/downloads/?a=view&id=3952