1. Get srcds and configure it for garrysmod (requires steamcmd).
2. Put the zombiesurvival folder in garrysmod/gamemodes with all the other gamemode folders.
3. Get some maps. ZS_ maps are plentiful on the Internet and the game also supports many other map types: CS:S, Zombie Mod, Zombie Horde, Zombie Panic! Source
4. Either setup a custom voting script or use mapcycle_zombiesurvival.txt. Make a file called mapcycle_zombiesurvival.txt in base garrysmod folder. Put in names of maps without the .bsp ending. One per line.
5. Make your auto-start batch file or whatever you use. The line should look like this: