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.
legallyinsane9, CounterTunes, XXXXXX, ☭, SergeantOptimus, Niko, Tassie Dingo, Skullfuck Commando, Monkey D Mills, smithc, xlimeTigerx, kETcHaP :>, BB, ZetroGames, Sonic1305, MrKrabs, MelonosGLua, Tout Le Monde, Sherlerk, StormyTerror39, Midori Gurin-Chan, Humanoììdd, JeezLouiseLemonMcSqueeze, Ignaz, XaToX, Roach, Gerardo, MioXD, CaptainFrelixx, TECH3_FR, UL| Martijn, Cherron, Sphere, (RGC)Mason, (RGC)Mason, Sho, 月幸, Nicholas, fazafreeman, TitoHub, DefeatenOnce, NovationGaming, mod36, Ryan, crazyian1000, ArSoN, Daddy Blue, Nameless, Lossy(Lenovo), ԼЄƖƊЄƦƤƦƠ ROADTOSILVER2 +71 others