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.
-=TOHMAN=-, re15revisited, Dan4ik1762, [GmT]Gremlinzone(U.S Army), kobolotin, galusins56, demonsoul, jose_19_carp, some unfunny skeleton, Crackers 2.0, [Optic] BeastTroll, =PP= 419, phantom Arki , Little Dreemurr