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.
trolmqx, jmike29, SuperShadowMC, gz-59, mcgato42, ShadowGhostTR, bellodomi @Gamager.com, Mentezif, ✩MasterArix707x_PL✩, MR.Gamer, anskima1000, CrayzMan [TR], Cal_1019, Grim Reaper, SOKOL, ดิ่งครับ[TH], Alex Mercer, artist1375