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.
bigfootufo79, bigfootufo79, 2567124250, lautaroriera1, AnEvilTurnip, BaranKarayel, <Data_Expunged>, seriou*****, RealN96, Agent Maine of PF, speznaz97, evilgood2010, 1111111, bbenjamin1702, jeroenwerf, evilhands, George :D (MagorCZ), 1228256275, anapui37, anapui37, BleachPancakes, KnzenoHun, ☣[DFS]jcash☣, [팀DT]포돌이, [팀DT]포돌이, GunScript, xXx_MLGNoscoper_xXx ( ͡° ͜ʖ ͡°), vinivallim