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.
bashir, dragonborn27, bflmn, alp.burakergin, TOO-BEE!!, ëRrØR Ĭό0n #NULL, neoricky26, ★Sp1rit★, gbudd11, daredevil4u, Tok'ra, EyE_RaGe, Kairu22, CaptPatrick01, [TS] Special Agent: Noot, =PP= 419, benmacxbox, Vox Maro