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.
thomas-kian, thomas-kian, jumahogr0, pabloemiliano2003, rmderoode, ✯ Kyouka Toujouin ✯, [SFS]NesquikScope, MR.DEVILL, Kdog500059, Sir Slothington, Nighti :D, killermano15, herobrine7720, Coleski123, heidemarie.coellen, RedxBull, ByPoLaT [TR]