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.
Unihors3, ramiroduran39, mossesx, General Dude, keoma1501, satriakelana41, vittorio.cadei, vittorio.cadei, thailandtin2548, cbothma, ivan_legowars, LoboBranco_, Frænne, wingslinger, [ E-Studz] -i1-, Shiro, Hunter, MarksXD, bielrey, lugame, lavriki33, ElBestiaHD Kawaiii, SplitPlay, senjakoma, RustardGaming, Morganator, Im_Maks, Im_Maks, Rudox, CaVeMaN, CaVeMaN, marius44422, [AGC] $pittal, Mr Cheeze, sophash, Sterling Pierce, CraftWerk [GER], Necrom, lucapromo, Lazy Boy (VietNam), Luxio01, basti7, Desire Snowy, xiongliangtian123, fellpopinha, Zexy Dex (no mic), BoaTBiee, benzengineer, >Neø Geø<©, Mush +80 others