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.
Mr. Dad Guy, HELL, GORYGIRL1996, Mister Cross, Mr.KTO-TO, eDp! Scumb4g, Chimp, ghost3215, [email protected], [email protected], Boogaclaw, AdvisoryCookie, Cor Angars, 178086057, FarhanHQ™, Michael "Noice" Rosen, jthmsonicfan, L4D2Fan13 (Birthday in 5 Days), Ruby BlitzKrieg T.R.I.C.kMaster, Ruby BlitzKrieg T.R.I.C.kMaster, nostalgia. #shitcoder, Warpath-the-drone swe, The Pancakeman 23, KotenokGAV, Stukakukaukaukakukamodi, meharryp, Rexxor™, draconica, draconica