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.
kriswertel, Arlanka, fedor, tizubogdan, Batman, mushraf_2003, Gokan_Nagara, MrGamer, BELAMAMMAR, JAI, benjaminl1984, austinamani, justincreator, MilliniumAK, OMGITSMOM, Stoned Apples, Kahldin, dragon789654123, hectortito612, TheChozen_1, ohmylordofdarkness, TearsOfDream, Zaish, FinalesFunkeln, NeoDukeNukem88