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.
Metaphor322, shdy_23, Maxxwell07, KelliherGaming, sv_fapping 1, chinjung667, LEWii, NGneutrino, SlyPony, El Piti, Igroman, maykol522, HandsomePalpi194, brentm.osorio, professional belgian drinker, professional belgian drinker, Digger Dixon, 피아바수, Brother Espenachio, Tai Lopez