This download looks like a save. You need to place it in your saves 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.
ilm9001 (QUACK), §『㎖i_CRo ζoFt』§ | busy, Mudchop, Koray // Fire, SimplyPXL, ☣[DFS]jcash☣, Tony the fawk, Karll, FireMaster, lonely19682005, RedTailedHawk, a proper madman innit, CMDR Renco, kyle.stokes321, As a kid I looked like a carrot, Teh Hatein, Ickyman, 감귤 작작먹어, x.X.RIDDICK.X.x, Стерлядь