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.
Strike305, Denied * Noscopeglasses.com, roite1, [Farmskins.com] Reijin, a Big Weed, tecneo, Deadon ( ͡° ͜ʖ ͡°), Dinget, Vinyl Smyk, Deko desu, Dr.Robotniik, Kilowing, Kilowing, Apex, Pigeon, How Super Terrible?, Red Hex, jjloney, luiz22nato, Negative Private Jr Chips, Fruit Bootz, Evaisa //Eva ♥, Snaggle