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.
mikhail.plokhov.93, \/endetta, Pétergaming[HUN], re15revisited, Sigalaeff0607, blackviper4362, RD[IZK Pvt P.] Pizza_Gaming, M4NS3LrEalBasta, Bakari black, Raven, muhamadalfikri_pesawat, allahu akbar, [TW]BlackFlame | 破坏性极大, wachis.17, npragland, Heckelex, Temmie, Mario44422 ヽ(⩾Ѡ⩽)ﾉ, Iucoat, •_Warsaw_•, FuntimeWithered3, assaltpl[RTF2], MapleOrnaments[F-B], engi[POL], engi[POL], ๖ۣۜBlueWolf, saki, TheCowJesus, LagTerrorist, eat pant, CaptainRed, QuasarBarkas, Jam, HiddenMonkey, Yew™, Lord Ghost, Uros Uki