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.
Fang, Danish_Weasel, yeah, mossesx, Yevey Gamer, blazerbrine, Jack White, powerstar2028, ArcticMikies, codename dragon install, ツ KevlarCZ ツ, /Ice/Storm/, ninjout, ansiritsa, zsopyryaev, Jerk-O, A-BsoL, (ISIG)Relago.B(#COBY), 8KingPower7, Metafire64, Dellsational, Schwiria [S.C.P.D], ima platypus, Waspbot, ToTsC 38, hitmany, Sans, vittorio.cadei, UrBesFren, Gato Robot con Hat, cruzsanto, Entec, lol, dan, dan, LOCKJAW, jennymiriam2512, Geshtro, Geshtro, Criz95, titacool1234, esteban.mijares, 滚Iunno滚, gun loaded with power, Medicfanspyhatr, elmaulicio, ElleAlexandra4K, Hal0Ash, aleksei2003shpak, anita77 +440 others