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.
Inferno_Hawk, misgames69, BlueBarracudaMedia, mossesx, zanofa88, Squmander, kgergo5800, SoVietDogger, wackyworm6, SwiFix, Zettazz, Blueblood, LagKittyIntensify, Foxy the Fox Pirate, Biahwtrooper, Starcloud, Elteeb96, SuperTJ327, alexisjohamdesantiagoramirez, Warpath-the-drone swe, Warpath-the-drone swe, Cä§për®~FYANB~, Transient Princess, LazerSpy, Sir Oswald Mosley, Soldier