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, Bunneh, FlauschiS, FlauschiS, ivan.andradeeqa11c, ✪ spoon, Xeno, EpikMythic, NOJ8, Maxchaos, [KOR]혼살InLv, [MXV]PsyKoO, Charmz, Rekt, Marco, =RGS= Mr.ROFLMAN™, andrewdbuilder, Yashirmare