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.
nanashi86, Snitch <3, wucash0, kajunjennings, Vassili Kirov, Tony Petini, mirniy.sanka, mirniy.sanka, SadPandalorian, ferrogabriele4, Rells_Parker, Nexo, bulloneria, SpectNaxz, Theous, 江泽民, JFK901, redfredhunter, shaungambz, AlSha, alcaloiddeveil, Flisking[Y], GeePee, Pixeltyphoon, Shade, JUSTDOITM8, MisterAlpha, Michu351, __Alexplays__, Raven-473, GeneralGraybeard, herman2000 [ウォンロン], gaming55.teo, re15revisited, oilasko, get gud, [TeraGeek] H.a.n.n.i.b.a.l.e, JonyAG, Giant Rooster [muttsquad], XenoNuke, Savagely Random, kovi2609, That guy over there, That guy over there, Freya, Dark, terronsitohd, Penguinchao, Xenoxia, Substance D. +432 others