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.
Kiwigamingno1, ZombieSlayer, tdbcat, Spanark ( ͡° ͜ʖ ͡°), darkplaygames, haichhan48l0, Ale99Ghosts_YT, Czer, TheRalle, -=Stabreka |CMBS|=-©, juwon666, Sadek, So6Not9Funny, matrocraft, Aide #Tryhard, jm030148, guzmancamilo02, Amityville1524, ali.sadafi2003, maximegame89, matiasovando42, POKEMOUILLE, hamzawwe619, Frankie_Boy, G-MON, asad419, PraiseLordGaben, Adamxspiker, EtherealEnvy, Cpl. Hade [1st RB], open8925, ED, predatac130, Leon Kennedy, juandiegobustos123, 50p, Yakibop, Edmond, Thalia McKenzie, eric, t3chl0g4n, Private Octoballs, Pork, Diggingwolf2605, Ahmed Jeddi, Captain Kurtis, Caevialto, tiagoschwinn123, nelson_law02, KrauseeAUS +16 others