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.
SC | Mitsuru, mindy, Noblesse, re15revisited, re15revisited, Dratytu, sebastianestrelladiaz, Tristwyn, 천수반야, rmn1981, Xeno, InvalidFish, Gabe127, ¤†Father Cabbage†¤, Ms. Potato Damage, brancomeister, Chilybones, [KOR]혼살InLv, Pacifist, Xan42, jawedyx, DesertHare, Church, plankton but in the trap, l.maire c, Devilswarchild, IceOnFire, Otacoon-Gabber, VooDoo, [NIИ]Complication, dirtpeanut, ๖Ͼᴛ Ferox, matt da g, Toon Link, ViktorKein, Nookyava