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.
Jon rsrp.uk, garza, Zurom, simsamsom129, kotel Emi (cat Emi), Thugnificent, Пахом Поехавший, DeSychronization, fisherhen22, dima.averyanov.2019, , DannyTheSkySailor, Douge, Douge, Duke, CARROT, FAZE corbyn, jyuitp, giangiorvanni, natanael.dph, Major General Ian, Peach, luckyandmamalita, Waffen SS Ball, F3lD0, csflores2010, LightDemon, HelloWar, falseprophet141, falseprophet141, falseprophet141, falseprophet141, virtminhon, desinteresado124, bard, TitanMnsM, BrokenLightBulb, Leuh Bykeur, Jack Baker, Jack Baker, Ilya Volk, [Ne'A] Sir Xoranes, Papamondo, emidret, Ihatetrainyards, Toxic I11I, Angry Pilot #TeamW+M1, Dimitri techenko, |DG| SaintlyDemonic, TAMT145 +20 others