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.
ComplexStupidity, Player, [p2p] S.W.A.t, ianice8102, vova.vetoshkin02, vova.vetoshkin02, sharkmuscles, OG Ambu$h, Kaora, Xanx..'Q~, Belociraptor66, Darky The Scout, Boy123467, NickyGeeee, raypalmer179, Sayee, gimiiii2701, mrloltroll866, Chriskraft9, Biga, navalcoy, Peppa Pig, Kowtang, yoshi31081, spoildchimp3, davidakiraly1, Fungisnot, sasaguy41, HoOdY, thecakemurderer, fancychu, Yoshian12, ddhs5845, ✪Hunter✪ | trade.tf, [VXGGO]VORTEX, supermanesd12, ales.tomse, Dragon112322