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.
Detective MAGNUM, Kinami, rmn1981, alexander.morga, TOO-BEE!!, TOO-BEE!!, TOO-BEE!!, That one Justin guy, tenchi632, 1bHC, Pruld, l.maire c, dirtpeanut, andreas.limsakul, QwertyTSecond, [CB]3M1L10(swe), Cupcake, Sidman, Vox Maro, SevaGlitch, Sakalauskas, gas helmet