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.
Cheba, Caton, vuvantre13012001, quantu497, CARLITOS NOPE F2P YT, sam_dkr27, sandro.tabatadze, Je suis pas raciste mais, francchescocampos, Antoniosantos002, Princess Daisy, saulroga24011998, unguru2013, OpticQuill KeyDrop.com, Diwilcer, Mr.Cat, marya, Пан собак, giankrouz, opazofelipe95, opazofelipe95, DanKiro20, haredurbina87, BMO, ryaan.sabri5, supremeinternacional146, Kirby, florian.deparis62, klocekgenialny_Key-Drop.pl, NarcisTheRed1_PLAYZ, leorebolledo07, gabito.gonzalez.2003, lenient rage, ʙɪᴛᴍᴀɴᴇ, 3KEN, kevinr81_k, hajonradzmar, Molly, s1mple, ramiromory28, danimewblitz, santaolalla2004, piecanny, teovanhoang, ุR, divashigami, giankrouz, Drubs Wagger, Adidas_魔法少女奈奈, Kukenox +221 others