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.
umutargavan2, J4nk5004 | gamekit.com, Psychosocial, Medium600_HUN_ c:, jacobic, michaeljohnathan, Captain Zed, Neplo 998, Brightness, SsjGodBeastPlaysVideoGames, heavy green Puppy, johnnyniuum, SharkFin, Lord Eoss, Izaiah Salter, The0nlyGH0ST, KotenokGAV