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.
Skroop, tomazung, Plaidiator, w21drakec, onlyimlau, ghost105239, DarkToonLink45, anmudojin, linksuper75, Kross_arh, nico.fox.555, 0thebrightside0, Soup Bowl, Sidali444, adrianpaez999, Klondike12wcs, belfim10, zonboso, Karpo, Elezeid, 0shy, Furryfox1418, SCARECROWL, Traitor, FeuerKing, darkwaluigi1022, Demonic_Terror, G-(er)man, doodlebombX, aray03, pufferfish56, seby_nike18, RickDracul, twolf, pepexelmejor, HeavyMertal, KEL_Na, SQUIGGLYCUNT, enesmustafabnBN, lxsferreira, dorian.sanchez, memotazo, Senpai, 오르는 사람, Thesidermaxme, gamercouple81, krishey, SowaGameR, magunkitti, Jasonawesome367 +219 others