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.
Cranks // Source Engine Fan, Akiba, Lord Ghetsis, ElectroStatic, Jabba The Banana, Jackblack, ramos.jc11, lolnig35, bbenjamin1702, YuriMental, P0rtalSh0oter, Pedromex2007, FS FROZEN SOLID, Catdroid, Zahilla, Bum fun shoot!, Bob Marley, Frost, Zenolisk