This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
Gringo, fekrtelyum, MozillaCheeseSticks, jonathan.boudwin, SMG40805 a shop, Nightstorm1003, Volaille ¯\_(ツ)_/¯, gaben newell, spellfresh, sansvamir, frezeer0021, Fireman123, bob, captainhairday, Rayan Moktefi, Colgate toothpaste, Sputnik Hoovy, Kipper, emmanuel54140, Th3Shad0wKnight #Sasuke, [email protected], A4MS, Sugarman, TheSpaceVirus, wonk, SunkyMan, Commander Toros (KPAvids), El pelon Blaco, XtremeTerminator4, Warpath-the-drone swe, captsnow, George 08, George 08, BBocianekPL