This download looks like a map. You need to place it in your maps 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.
Dijapter, hyssersp, Team Static | Smiley, Niko0102, ChalkSauce, Takima2001, londonskyrim, fazafreeman, SuspiciouslyAwesome, Starwobble, i <3 chumb, blackknight32, volume1539, Very Spoopy Engineered, RobersharkY, Magnιтυde, ytp123, [RaM]TechnoScorcher, ronaldomrnd, ||| Mr.SFM |||, ginseng, C:/Meekl.mkl, Gamus, Frost1551, LilDevil38, BLaMotte, Icewine Rose, Icewine Rose, lonely19682005, Andy