This download looks like a save. You need to place it in your saves folder inside the Garry's Mod 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.
Skylioz [Uranium], LeVraiGuren, ino, Gamer569, recaaxd, clarkleenderts, Samson, trexgames11, Pyro Gaming, KrUsTy, Wellan, gustavogillen2109, farisaryan2011, durbanilove, trowquin, david, david, bia.barbie1988, edanrolu10, edanrolu10, Quandale dingle, yopse, entechentainement, Mauri_fdz, -asddddddddddddddddd, _civciv9090_, laguafrancisdominic, amaropontigolollol, 888海日人`,,`, Moussa, gianbasilio.nisperos, Denudels, DARYLLY►☻♦, jalmamufyo, jacinto, slavitek, a, Christian faderan, Lunae, oLemonn, sedatsekip8, arturoagueromorelarturo, brulautipros, arturoagueromorelarturo, Sinaloan pack mule, ChromiusJ, PinchorGetta, 2EZ4JC, Je suis pas raciste mais, Hojj +406 others