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.
zanofa88, jakobgustafsson.1995, decadent_uk100, 4x20 uguu's~, AWESOMEKID1214, Digitallyawesome2, KingGrim, (╯°□°）╯ The Rage of Silent Jay, Teegee, Американский убийца), DaBirdman42, Bhhenry, Culture045, Drem, park smoke, park smoke, park smoke, andrewdbuilder