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.
frutinha, Atecs, SkepticalEye, xXFamouzZ, Plebiangamer, TCN_Floppy-KatjasPizzaBote, The_Cleanes, Pablurp, DRACTION, Zink, Out_of_the_Blue3, Radiolar The Pootis Man, Quatz, gabiiplay, gryZor4000, gryZor4000, wiesbaden203er, wejrom, -1, seonbinchoe, Temqe, Jodie, Ethan, Meido, Scritch, 1Dogg12, SuperGamer23, A KAWAII ANIME GIRL, Santi ARG RIFAS CS:GO, lkeymayoral, Super Jumpe, arcor777, clarkorino1, trolex180angel, sotirovits, Sskewer, echtmerlin, FunPlay, Mentrooo, 2564001131, iyrii356, Atomic Legend, Danny_Strong-DE, Parker, brandon, hankhank0202, Captain Kitten Fart, earthfase, J., NaegiGamezz +106 others