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.
[DC] Glennartz, garrettmershon, JellyWaffle, TxRx, sss, khm060608, brandonmiville68, sperooa, [SNS] Xwatch1009, bflmn, TheWolfSVKgamming TV, 710 Leaper, dazpowers, južín, collingray0713, Kawai :3, azamani0326, Abrackck, halflifeunlocked/unlockthexbox, A Deep Commitment To Konata