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.
quocanhlaem, cedricperez500, re15revisited, Mr Soarin, fym, gabimagloviti, sshahrabadi92, busermanov5, DJ-sona, hank555, ~~Snorgan~~, Ghost, flameroy234, vinivallim, Sistine, FutureFox, Zokrau, DarkSniperTR, ikosmid