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, jorgenieblas, PoWn3D - Über Skillz, LikedGenie853, Framzan, Framzan, Barry Burton, jdhr24, Dr.Luzi29, G3enius, gloowb, Cone Licker of Sponge, Googol, l.maire c, YeahDoodCzLp, BaconScout42, dirtpeanut, NEO_EX, Filtershakes, ParaSite, Zom3ieKil3er900, jdramirezelmo, ocodad, andrewdbuilder