This download looks like a map. You need to place it in your maps folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
ImHADI_, Hunter, toxik.gb, Zombie Stoner, marto14_1998, ƒσяgє ѕ¢яιρтι¢, Protey, Ethaligan, Snowy, Who That Be, ClaytonM456, development now!, Jose Cuervo, Babou , NawNaw, Killer, aka2aka, Mr.ROFLMAN, Free2Play