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.
WYDRA_GAMING, Ds1000, Tributon, bashir, kgergo5800, kgergo5800, NePoBeDijkMiY, trudyhughes1, bigfootufo79, BrianHorror, mirniy.sanka, mirniy.sanka, francogallardo2006, re15revisited, re15revisited, patrik012385, Sans, absza15, isaiascasm54, marwan.chalaby, TheShadowMann, judith.19.jmr, *****ed, leonardodente_, couldntthinkofausername1, couldntthinkofausername1, Tude, radik_melnikov, studeter, gyenge, 蛋疼的一天, TheGoodSonJingus1012, TheGoodSonJingus1012, buluthan_22, TheIronGaming, SumThorn, errollees, He||oween, densi, jeroenwerf, tincho_22picon, Marcel199727, Ninja Killer(た.き.も), aleksander_1999, BillTheGodOFL4D, [SG] Mad Dogg, pro125gamerbg, Freddy Kabusu, Reed Timmer, berecz98aron +12 others
