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.
shockwavealex668, DizzyTJgamePL, HejtUG, jasondelacruzbengolea, dechev_97, ♤Isaac Clarke, Johnny, marcin.juz, chevyhorsepower6.0, Aunt Phatso, THE BONELORD (krunkdinkle mcgee), Tabus, hgji13, LDLC T»»¶EAM