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.
Revelator101, [Empire]-Xx-Zombine-Xx-[Cpt], [Empire]-Xx-Zombine-Xx-[Cpt], erayesendal02, Martindob, eraycan02, babaarda2005, ⒽΞ⋀DR00Ⓜ | CSGetto.com, falseprophet141, falseprophet141, falseprophet141, Ventuhr (Ashen), xzafercan, rmn1981, terminato012, nabilyusof, Nova ● Peter Parker, Valkyries733, BomBerMan(bbm), YoURMoTHeRSPeNiS, turkufiliz, (TR)Bozkurt, RAELIX750, WhispeR, Ronson, Ur-Didact, nesguy229, Blue3eyes, just me, lonely19682005, remarpc, Jd, NotWaitMe675, SILCO