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.
tjpetpetty2334, Red1, 暗影先锋, daswolf, MrSuicidalSheep, Kirbysqueaksquad123, moartea_221, Spooki69, zibidi26, Saysay, 33samiyunus33, pvpvirtuozz, Last_Unicorn_VK, encarnabra73, Sir Tony, fromentvalerie, jm.jason177, Pawu, adrianhq, mishalokh48, justing56, secretyaseen, ajdaniel45, QTG坑, MAYSeven, [C\P] Dlopper, Saysay111, hunilex_07, the1andonly54321, nicoconda, DavIsi, CrazyHandz, Davidr64, George :D (MagorCZ), SubZeroEclipse, nickmurry, pokernightpros, peter2355, rapooe, rapooe, rapooe, rapooe, rapooe, cxxx_71, cxxx_71, newonhuu48, Kaigit, ninjadad36, Pixcraftgamer, haopham863 +18 others