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.
zakaria01af, captchcrutoi, Aztec, IDankI, apcmods, noaccess, Speedy, John, DankRP Memer's 4 Life, Hikto, Moses, CheoneDev | uniforcestudio.fr, thehyrican, gryZor4000, Scorch, FrozenY | Christoph, 2050398990, denomito, gigamgbo, mariussa11, minituregamer101, mnami690, AllSchool, .:eXon:. PB [HUN] GoodHead #Pyro, Jardeen, Braven, Maxouuuuu., Maxouuuuu., lilyryan58,  Nick M., mattpeckron, angellimon2178, Mr_Fourth, jakecannon5, Get_Right, Mr Cat, Senatu polarservers.com, Senatu polarservers.com, DENUGAN, ❤ C R E O N I X ❤, ❤ C R E O N I X ❤, Myles, Myles, WRIGHTR.2017, cjeje84, DaRk F!R3, DaRk F!R3, [Sn!P3]Boub66, ティピドル, Serbious +33 others