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.
paoza1155, simark558, simark558, simark558, daavid_enriquez, DerpySlurrpe, L3M05, SuperSegaMario, SuperSegaMario, cupcake101, Im [email protected] F33LING IT!, ezequielmorano, =PP= 419, nycreddog01, Sokkah Abdelkader