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.
aionn, Drake, ataraxy, think fast, 03kj20f03232, Hybabyba, elliottserver, BiRMiK, Topic™, Stanleytive, .:|Unknown|:., Pancake Jenkins, Dr. Bangaroo™, Tymba, 乇ㄥㄥ丨ㄖㄒㄒ, zezmiclegend, Yam, Petski, kila58, Valids, KeksDose [Trumped], ElectricalGaming, hfhfhj, Excel | Trading, Nightmare, nidal el makdessi, Strobez, Joni, Tails, DamyanC47, Neko, Gradualsphere07, YanGo, windows user, gogenbI4, gogenbI4, MofexGamer, Cave, *RD*Kr!eger, maria_silviaj, TIGER, TIGER, Amber, Jonees, ozike21, L-, Bulet, Bulet, Crapy, ahmad.sabbag +135 others