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.
arayafelipe623, Player, mohammadshishany, re15revisited, rongta, rmn1981, [Sir]Sn4k3, I Haz 3 Cookies D:, martin.nkv, Taradosaurio Rex, Der Maestro, ARTiiK / FiRE, Pew Pew, [email protected], Nick Hertz, O'Nesix, -vG- MaZe