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.
aidan9030, supercreper111, Light-DelaBlue, ga-henrique-, Mentezif, grar21, goldstreet101, proli2000, Asher9, [green] Click, Rtkz_wasa1974, General jack o'neill, Therealflashman, kawaii anime kush lover 69, Requestking, Jimmy33650, MrKuBu, kyle.stokes321, kyle.stokes321, Starvox, Селёдка, [LCSRP]Lydesko, [GG]Tense!, Razor