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.
Igual's Coco, KK || Stormz69, [SGN] 0xymoron2, diamant656, paulm98, [KOR]GOM, gamergame067, kxstry-., HELGO KUNG TH, ๖ۣۜŦØ, Roovle, ☣芫MagGriffe芫☣, TwistPlay, Zeon, [IW] Hodyrev74, [IW] Hodyrev74, ira-ki, ZNAKE, aleksansander, Shonipod, idreezo, Itz Elite, Waren, Eaglishee, Amber, [RGT] Laskae, Btc1322, Grimred, Revolving DCON, Mike, El Dugget, Rubik, Assessedsumo643, Rire, Rire, zfadarkness, Mason, LightsoffGaming, Tetragromaton, Shotacat, Hanleyjaa123, Straken, Starwobble, Atexsor, Flattershy|CHEATER|, Flattershy|CHEATER|, PandaGrade, |★_ƒοмά_★|, Haina, ZaZa +23 others