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.
h3nNiiii, joelgerman2112, joelgerman2112, tdbcat, serkan_guler18, abm2, ariano113, chinjung667, Koopashell TMK #Meem, QTG坑, Dutchman, ๑۩V.I.P Shawn Michaels۩๑, [FP]ICEMAN, nasr1355, adamrosloniak019, Czerwony, ice7777, ardhikaraharja, Blaze, chuckfancy47, suarezviel, NormenXD, petroff.artiom2016, Hеrmit, (MLG)ShadowSniperBoy8418413, Pilot, adam.fakhro9, zxskillerxzcxz, FiLzO, Black Coffee, TheMaster, dragonmaster99, dragonmaster99, MarioDox, xbeastguyx