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.
negi524, com.antilles, DedZed, Commander Con, Ali-Alastor-Sans, erik.pimbor, Rafa 'ᵜ', gunersimal, Reiten51, Saru, bahamondespastenes, taratap_, mktnazar, mossesx, Yaya, Makarov463, Phonrage, KID GASTON CHUPAME LA PIJA, keanec73, archicastor1, dasher132, szabomonika26, soocool11, Boss_sila2547, LTK Manix, Satael, Dan4ik1762, Little Crumb, ArkAJBoy01, D3VI7, mjh9994, ghelt2000, patatex111, Clever Name Here, falseprophet141, Ventuhr (Ashen), rmn1981, NOPE_NOOB, JAAY-BIRD, 9, That one Justin guy, robbed by a chicken, decadent_uk100, Mreaster01, ☪.Space:Wolf, xinzhida321, julio123457890, Liquid, algaenvip, MegaVector101 +21 others