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.
ray545770, ericespinargomez, NickFuchs, Happy snacks, модный орк, Risen_from_the_ashes_[chief_117], Jimey, agucas95, Xx Anarchy xX, BorysCSGO#GG, vera.yna8, sergiomagana2013, , russ, Prone, dunno6376, Blazingsteel64, alexsuf, kida0208, eldridgeedd, [TBF] Nightfire1989 [GER], yeffersonbarros27444, 14fighter, vannismith01, TheClever1ELSD, duckytheduck024, Grout, slrfm, Ronnie Paints, Große Führer Tobias Sevcik, Mako, eliseev-valyusha, braun1070, Redy, hunterxy, Dark Telamon, Nksakos, jeremywn4, JustMonster, Nemesis711, EnderGuy, cKrisis, manoelmessias.j.santos, aa8326138, 【† -blackskull12321- †】, SirSpektro, ppz, nolanmartindale, goran.rakovac, wohlddf +58 others