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.
VulpinnNaifese, RyanTheHobbit, AlecSlender, Marghe, SASUKE UCHIHA, Rafa Elfasos, Whiz181, Floppymonster, cuevasluciano1, MaitreTrollface93, igings, Bill Nye The Science Guy, Gush, John Caprino, A$AP Yams, lucas1128, palitagaming, youssefo, Nabuu, xtony71500x, The Inspecter in horror maps, what am i?, OG l sawyerzstyle, Link, TheAsianDoe, bourguignon549, bluestack65000, Anders!, Fuzzy, [SPD] Josh, Brisbane—Destructo, .Exe, KHALED, ishaqi, Frisk, ImFaiiRy, Augusto Pinochet Ugarte, TOBY, [Béret_Vert]dkylian, Zapster, bitch, BaumArmy | Titan ♬☢, pandakawaii, steveowen528 | hyplex.co, DradaL, Solen Frechette, BySheiiK, Nick Fury, MsTeRA, ky-mani01 +499 others