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.
srdylanedits, enesmustafabnBN, rfong37, slawek.witkowski, Ashir PL, matoulechat2, Koopashell TMK, Sov, Dimitri Petrenko, turkufiliz, Lord TL [Hun] ☭, Proder, Commander Toros (KPAvids), Foxica, Mad Matters, ©fabricio o ouriço, DecoyofChivalry, TF2-Reddpie, Warpath-the-drone swe, farra222, orlandeloscientos, 實況主 TimDocHt, A Deep Commitment To Konata, Dis Dolphin Guy.., Dis Dolphin Guy.., Designated Kitty 03-052, The Unlucky Medic, The Unlucky Medic, ruminaas96, LazerSpy, SevaGlitch