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.
legodavid77, s20023995, super250, MaceWindude, bta678, [SOF] GalaxyS7T0P, Edit, Edit, TheProHaxers, [UNSZ] slon, 대아온, [teal] Kawaii, Believe It!, Hubert Joly, danilstarbird, AmaryliZ, ardhocool, Codeine&Lolipops, ILLuminatik, hoya7150, chaseplayzgamez, AppleWubs, Bobito Pawner, rlz, Cedjed16, Cedjed16, Cedjed16, Cedjed16, TheDrago, The Golden Gamer Official, dexter642, dawidosky1337, TriForceofAwesome, Kagora Uchiha, cpretorius539, trondkvam, luck_the07_gamer, OwL, [IW] Hodyrev74, lil'Chinese, Royzourboy, eli.joseph.o, Sufficany1970, ivannu, neto18sk8, treasureeagle90, DarknessMC, Death059, Max1969j, DJHOB$, winterskywere157 +145 others