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.
JedrickPlayzYT, u_vinnik, cocode2002, SirMonchu, crungEEEr, TheNightmareGamingRo, sabaidee2514, Lock [redline.gg], Sanal Oyunculuk Abidesi, Mr Genji, [TF2]Handsome丶源氏, heartless.oppo, MIster_Toro, pucmail, Scorpion, Beni1384, eternalkuuseki, SeniorBanan69, sami.asghar, darkstrikenoir, ชมพู่ จ้า, virus, ^4jakey0804, mariomario1748296, Toy_Bonitto, bestball1994, state from jake farm, BoyWolf300, o0pe0o, Qbill201787, znhan114561, luciano129, cruzsanto, GojiPlaysGames12345, GojiPlaysGames12345, asaasin [Hun], Live_togo, butabuta19721002, lol, lol, tester0987, qwe_ert_02, laowil, Tarte<3, YugRafael, David Vazquez, lukamarasevic, Przem0THEBEST, pedresilva4885, visanechipa11 +1296 others