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.
Hopeless, kriper200459, ErIk, , Dealer, Velsharoon, Пэрсик, Наливной, Da Rockwilder, matematica_FamouseRP, NOAHBDS, Wyrzutek, Justin_WrldBoss, John MacClist, Dreufter, 3efirka, lorenzino09, , lunextek, BösesToast[88], Kroko, SirDerekHudson, SirDerekHudson, Человек, lanco0, The Fruit Knows, huy233, scammer, Prismic_Blue, gamercy095, UndreTx, P.T.U., MrReload12, НЕ ДЕБИЛ, EdwardRB, fox, fedys, fk134679852, CheeseDem0n, Invisibledraw, StamBlyat, Guderian, KungFu Snake, (-) , BPGT BlackPhoenix [Dralion2k], [LOS] ThatDani, MarryBye, Maj.Kat, turjsakinaz, A British, Japo Le Ouf +930 others