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.
joshua_robert, J A M E : Unlock me, darkshadow891, loissous17, corbindevrick, KQ, zennat.3195, [DSG]PedoBearJoker, makytitan023, HOI I'M TEM, Rubixiam, TG_Officials, PesaThaNoobz, BoogieSVK, Mechno14, ヒロシ-高橋, marce_368_2010, Boy123467, wuzzleutd, HoneyFerret, theblack702, theblack70, Tolga63430, Flamalam, 0adzie, CheeseMaster-csgostakes.com, RIC, nucelar, NexusGaming, bettysiders12, Nafiblu, PATRICE WILSON, techrobot, FreeWill99, Bendal, Raized, jellevanoppen, lucdop, MrNemesis62, doing your nan, Deaf Lone CH, AndreG4mezHD, Hobo4President, Scrub, Crazy1012, sezgincanbozkurt, vahgonS, SINOAX Sacrosnact, Use More Gun (UMG), mitchelljones31 +296 others