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.
xlunativ, vip.210, [D-D] ZoMbiE13, sijiloro08, re15revisited, re15revisited, 날 아냐?, patrik012385, [SkyC] -Likey, Gasomi, bernardofoquita, alejotriska8, noahlord, 无痕6666, [D-D] Looter, wangshiping3507, /\_/\, drunkenSocrates, .::S-RaWeN::., FiReBoSS, BLACKM3SA, l.maire c, andris.glazers, =PP= 419, DrakeAcheron, god85000, julielaurent1974, VanossGaming, Mr_JAME (TH,คนไทย)