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.
Shiva, Franklin Richards, Franklin Richards, peter.a.zafiridis, nnabilchiban01, iMatrix, Anonymouse, Warpath-the-drone swe, =ΡΡ= 419, Edd, Hds46, Dookatroopa
