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.
█▬█ █ ▀█▀, jacobvo06, georgevladtudorache, jacobvo06, randynipaul1, Sonic The hedgehog, kbubuka, Polloteman, kolano, Serjikjkeee, HeadVIPShot, BOOM!, rene.montero878, nomerxx123, cjthebeast93, emil470019, colecipra, shea, miguelmadrigal274, MB256's, nassim89, fernandobmwm355555, GAMMA_ALPHA0, GoldiePlays, printer83mph, Gagnetar, ★Silver Lake★, CONRECON, Kolbi(HUN)