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.
Atomic Cookie, jaserdavidson04, TheGuppy525, MrRyanWyness, amars7590, The Conductor, bird, [S37K] jacobsf, 게게임임, Døx, DharamAsh'rak csgofade.net, DharamAsh'rak csgofade.net, Aaron, SmithY | Pepsi, [ᐰncient]XTrollMaster, ダークフレイムMaster, Lloyd Subwoofer, ConManRoadz, Sauramel, Fb||DarkWitch, ZeroMelody | kickback.com, [red]폴[blue]츠, [G] ethonickiller (trading), [blue][LFG]LittlestSweatshop, Heckelex, PANDAA, Serial Killer, ♛✯Adam✯♛, brin900, EV_SkyFall, [H3CK] LordThunderGamer, §『㎖i_CRo ζoFt』§ | busy, AUGamerZ, aminesaidi662, Bagz__Bunny, nagymark696, t҉o҉x҉i҉c ҉┃11┃, mega_joules, mega_joules, Wolf Pack, Jan Tuck, Jan Tuck, Jan Tuck, Jan Tuck, GTAWeazy, ﴾AIRBORN﴿, Teal'c Jaffa Cakes, Teal'c Jaffa Cakes, Teal'c Jaffa Cakes, Teal'c Jaffa Cakes +68 others