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.
re15revisited, Aint., Hazzacus, zennat.3195, Atrocity, ImHADI_, Zombie Stoner, Connor™, 89Mendo89, ♥N-G | Mr.Friendly♥, [AST] Aracnafaria, development now!, Jose Cuervo, ✘r.i.p✘, [SkidSec]SnowWw, Mr.ROFLMAN, Fi5ve, Silent Bang, |WZ| ZeRo