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.
muesli_grey, sebastianromanelli9422, alexismarzano54, Crok Dok, 2021396010023, clashwithpekenoah, tylerdeondoyle, aniazyjewska1, cobra4148267, princess arres, doanhnguyet100777, ☦️Елена Кошен☦, James Cameron, Iris #SMURFIN', Military721, muche, nikkee666, Gerwazi_Hodala, MAXQUSEK Key-Drop.pl, itaygal1104, NarGIAZ, colehines2, maxielprimo47, agabalik8, Kmdr Revan, kaikejcd11, Depressed Furry Friend, ameen--[TH], Unforgiven, ILoveEggRoll, kunanonoradee, jaspreetsinghbansal63, BaZzi, mossesx, RoaCh秋月, preampcr, VictorxSqueeky, rohansmith871, kefir4uk, FR0SST, maudf07, hs3pya, Hershel Carson, datkhung68, QAZ025884, silvasandro26061972, TOT0886250470, kangur845, Bomba, Leeroy Jenkins +36 others