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.
FightForCountry, Adelin^_^, Articlegamer, kondzio5694, ︻芫═──, garrysmod1300, Bengt-Åke (Norway), Bengt-Åke (Norway), Camper, Spartan-627, silahli2003, cacap227, !rJ3$T3r, valuablesilence56, [ANIT] Handcuff Man, PunisheR, Heckelex, WarNyde, Frankie, AugeLtu2017, Wolfan @GamageR.com, Exiled, Exiled, zackbroad0909, Googly Goobly Cat, Scared_Turtle, Colonel D., sℓαүεявεαsт12, الجندي الرديف, Beşli_Altmış_Üç, InfectedCowBabies, KillerPunisher, Freeman, thunderbird7601, pz38na, Cashkingb, ☢NuclearAtomic☢, Jack, _Murphy_, maniagojared