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.
TamTam, キラ, Vali Lucifer, wap bap ba dah de dah dah, Hinweis, N3m0_Jenny Tull, thresh815, Matze, Sage, Radioactive738, Leon_Kaneki, Rambo89, John, John 1337, [DOFZ] | ACEHD | [GOD], LxP | Shadow, laoo233, wg15, GameDestroyer999, vanesa-arga-aras, oliviafansub, Mlaenu, Patrick, CaptainJack, Dude Animation, Reddok, NOHA, MATT 64, [AG]KydZz, Reissackhaar, no.za, athariqbal, The_Unlucky_Guy, notMini, Mipsmops El Fetton, brycedavenport707, Twitch | aFreshMango, [BuTzZ] The Animal, jonsch1337, xPotater, 95XTX, JohnCena, Merlin, Painkiller, Axspeo, Vailur, Niglet, Neraxor, The_Walrus, g!ml! +5413 others