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.
TheMasterCreative, Gringo, fekrtelyum, ae.sese, MozillaCheeseSticks, jonathan.boudwin, SMG40805 a shop, Galacta-X, Volaille ¯\_(ツ)_/¯, death by salt, spellfresh, sansvamir, frezeer0021, Fireman123, bob, captainhairday, Rayan Moktefi, Colgate toothpaste, D3f41t, Kipper, emmanuel54140, Th3Shad0wKnight #Sasuke, [email protected], A4MS, Sugarman, TheSpaceVirus, wonk, SunkyMan, Commander Toros (KPAvids), El pelon Blaco, XtremeTerminator4, Warpath-the-drone swe, captsnow, George08, George08, George08, BBocianekPL