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.
Weregriffin, Mikeyeduardo08, jwright159, mossesx, tolf-dietram, Atropus, POOTIS45 (YOUTURDADMIN), uriel_pablo, J4nk5004 | gamekit.com, Isaac Perry, Polskitrolgaming, kaoru124, NI CHOSI, ccipra12, colecipra, LINK2ChaoZ