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.
Berenost, 7-Grand-Dad-2018, (FFS) JORDAN, TheRoyalBeardo, Night1, Take My Ink #Penn #Duck, Fluffy Bear, Kemphler, [Z]ikario Bolt.gg, PointKillFox, kayral59, kwn13, The_king_core, lordesdogame, ASzaz ^.^, ShuN, Kurt, ShaeOverRated, McKay Medium, HowlingFear, dandan, Demonn, ricsi1160, |★|CaptainObvious|★|, You, misopsychia, Manoz, stafffnight, Acnologia, FrEaKy, TealAntlers, Melfi, French Terminator, Small Vampire, moisespe05, Lamborguini, FieryZore, booper dooper, Aranzou, ColonelJohn, NoorbbY, ass whole, Syn, DarkGnome2, Darkmind117, ★AdameK★, AminTheMar, SpeedyStriker, LePandaGod, Hyperiotic +951 others