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.
blufor, Bhhenry, TeaPartyBagger, Delta_135, MrGeekabit, Bullet, Captain Nurbles, Ho Lee Fuk
Submitted by garrysmod.org