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.
Gerbinio, Apex, RoiLinkVideo, ✘ NeedForWeed™ ✘, idp21113, CrYpTikZ, frebourg-wil, AppAqs HiDen Title, marintostivint, TheMasterCreative2016, Xgamer63Gaming, Napp0 [SmbH], Кирилл, Twitch TotallyQuantum, Soul_Slayer, Jaer, lordvicor, Maitre.Yoda, Deliora, MrZenith, Pandiox, qxtat1234, Hanna16, boplol123, skynext, AnnibalExHD™, Illegalcheese, Aziz, [RLS] Kreativ | Max™, ps482316, Moriarty, Sjean67, mzellebulle66, R1SM, DildoMan, NAEROX, Graped, Synx, Aroxlist, Dylanamo CSGOReaper.com, Alaska, Azerty Gaming, Donut, therafaletigermeet, TheTerminator, douglasfelicio193, Hastein, Powernity, allan.59850, Bane Sauce +66 others