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.
ThePiter22PL, anony.mous, Ashir PL, Uncle Doge, Medicilla, [email protected], Shwartzenagger, OH CAPITANO MIO CAPITANO, 3rdHalfConcept, $$MORÐ!$$, BlueB1rdie, SirTalis, ☭ Lord TL ☭, ☭ Lord TL ☭, ☭ Lord TL ☭, BigWAT, BigWAT, DecoyofChivalry, Warpath-the-drone swe, Minimole the Lorax, Phenomene, Ixelf (Angryman)