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.
kulich.adam, Gaulex, RailFan9089, RIP ACC - VAC SKILLED, lil'Chinese, Almighty Strider, SeOl-LiN A.m., ejzp_1999, Eoin, NoGame|NoLife, Frytak158CZ, Tony Slark, TazManiia, me gusta, Shadows5815, Matyas, liebert265, Speedy ^.-, 알파고야, Retaliation WTW, nabilyusof, elmoiv2013, Ø Mr4Prox Ø™, zarbon5000, 과부하, N3dRo^, tylerrobertknight1273, lazymoon, l.mestanek, SIooF.☆Geck, Goigle, poopy john, Shifty P, gary, Brosef.Stalin, iamdaaan, Coincidental, MGN | Spacecode, cider, eletrolitico, Gamma Photon, Hazuke, ChrisspyB, JonGamerX, PawkoW, [FR] Tiboxxi, AlexXx619, mark_xero_04, Catastrophic, SamSurdo +38 others