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.
dikihamka, J.Raul, amyrhsynn, TOG | Majorkiller, Predictable Manner, Gordon Freeman, BAST Brushie, The Sink God, |Konamichs|, Reed Timmer, efe.840, The-Boy-With-No-Name, roki31, roki31, boblealeonard, boblealeonard, ^15 Enemy Sinon, ikosmid