This download looks like a lua file. You need to place it in your lua 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.
SeNNoX, silvasandro26061972, therayninja, kawanssilva10, hueh20424, Vincent, Leprodada, SyncArts, , rogeliomolini, YUKIKO OKADA (ISIS), nate.jones2001, spiders2222, hades ★ reborn, Proudnoob4 #stuartlittle1999, Crow, SMG, nikolakiller111, amirsalarsaberi, [Louat]Neom, Strike Basket