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.
applws563, deivis_salaverria_hernandez, Family_wms, Kappa, Microsoft™ Office, ✪Qlutch, seedetail, Slo, Feastius, Thomas10152000, Streem Shot, Marsden, shawN, (T.E.V)ItsLiam21, DasBoSchitt, Lt. Lobster, Pixxo, Xx_Hunter_xX, MrFucker[XD], [BOT] TMGFOSTER, KaosHeaven, [NWA] Cat with a RPG, Koray // Fire, Gino-Mad, Shirino, The Pravus, ZeWalrus, ⊙Tyler_F_Hunting⊙, Josh, Adoring Fan EZ Twitch prime, Vokster117, IcyDragon617