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.
-PG- Apache, Smore, Ano1010 Slenderman, re15revisited, sebastianestrelladiaz, TheRealNerd™, <rainbow=999>MEME</rainbow>, ⏚, JustPlayNL, banK, madmaney123, Hyper, The Hotel, The Hotel, vasgolem, Prayuth Jangocha, pek, Melo675, DoctorDib, marcus_mh, IceGodLike77 (G.E.W.P), Kyhl99, Sergi, hawk, benbee13, Jurisdiction, Who That Be, Jake T., Hunter2432, grantdg1989, somebody in red suit, [BC]Biolock(DPG), [RGS] Martino, NahimBlack, Didgeridoo, sneekylilsloth, Echo Virus