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.
☾, ratiweuszekk_ Key-Drop.pl, taptlue007_TH, benciekKey-Drop.com, italusilva90, Ghost2078, MaDaRa68100, lotubotluk, | OZYT | OĞUZ |, jeironfkl13, tki92032, cartellowrider, nattapatdin14, Demonic Assassin, prOsto_глюк, Noah, sauserange, ivanmitenckov, danyuwu10, Pyro, oreganazerty, Clayboo, emartineslaureano, nurettinmurat90, tutorialesypartidas10, vlad.sem2222, klocekgenialny_Key-Drop.pl, Krt3k, jameshj84, Zeka, Varga_Dominik, rh602wss, jojojohn939, Default_the_bro, davidlorincz12, Adex452YT, MZN hellcase.com, Anorek PompaTeam, browan53300, madval26, eduleandro4, GORİL, Oscar_bmv, QUEXI, dante.1925, SonGokukid, Senpai, unstercrafter007, Nekyto, miskoltzy +746 others