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.
miami1472, Zetreve11, 18rob, hl_cs_dmc, Wiikhou, Cancerous, Edmond, riki_mkt, justmartka, Deeckobraz, Deeckobraz, omy829, omy829, omy829, RafaelSalvadorWheatley, zombiefighter13, Puke Hammer, ThatSmilerGuy