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.
franciscoperezzz, sodatakumi, Zaddy Zayy75, Mawin Lew Pewn, Dwight Schrute, ☆EzzySkills☆ #Road To MGE, carlangas_121, руслан, Adamek656, Aimshock︻デ═一, TobiaszPL Dark Force, MOLDY OLD GREG, Vushaiku, Mrkrabz