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.
LordGamer-SAĞLAM, alekseybeker2018, Swern.Gogh, thailandtin2548, TheNNexus, trolmqx, adagiocheese2000, jmike29, SuperShadowMC, gz-59, mcgato42, ShadowGhostTR, bellodomi @Gamager.com, Mentezif, ◆MasterARIX◆™, MR.Gamer, anskima1000, CrayzMan [TR], Cal_1019, [!?] Gamizard, LeLennyFace , SOKOL, ดิ่งครับ[TH], Alex Mercer, artist1375