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.
sapegyn, Detective MAGNUM, lysikov_valera777, GamePlayTactics, ✘ Wyzzer ✘, ☆☭Sam☭☆, vlad_mom, Ollius, enderman62208, lf1498, falseprophet141, raphaelsalzwedel, ♫ ۣۜSimon ۣۜZimon ♫, Spunky Mei, sygjh, We Shall Go Together, Elena Raven, Sneaky Snake, Randoug, ShanManTV, FearlessEagle, manfromarz, Flaming Badfinger, justincreator, jeffthekiller96, jeffthekiller96, for*****, Scardy, SniperWolf, ░▒▓HoverCat 0768▓▒░, Bayne, ruppfive, Dr. Dave Freeman, Infinite Light, Infinite Light, LOL DUDE, DarkSoku, TheAsianPandah, PR3D4TOR, Lord TL [Hun] ☭, Supreme Chancellor Angelies, creative oyuncu[ASİL TR], Dr. Ermac Freeman, roblewis200, myspam106, Riha.Jarous, iLumify, BLΛZΣ, Dnjido, Minecraftman641 +55 others