Not a lot of wave/timer based Z survival games out there, these are the best I can think of:
Killing Floor series is pretty satisfying. It has waves of mutated monsters and a boss at the end. It also has slow motion kill effects, it's pretty cool. Can be played single-player or multi-player. Probably the closest to gmod zs there is.
No More Room in Hell is a source mod survival game where you're thrown into a Z infested city and have to weave through streets, alleys and buildings to find a way out. Single-player or multi-player
World War Z is a game based in the universe of the world war z movie. It has a storyline with super hordes of undead. It also has a multiplayer matchmaking system with various game modes, one of them being Horde mode, endless waves of undead. (currently on sale 80% off)
State of Decay is more survival rpg. You can find and recruit other survivors and use empty houses as bases. You have to scavenge for resources and keep your survivors happy as well, but the leveling and crafting system is quite satisfying.