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.
fuggettiemanuele, hskjhgfj, Jolt71, Da Cow, HG | Talbot, HG | Talbot, -hg- Porter2455, re15revisited, painis bacon, DrKazex9, Koala Whisperer, HG | Great Leader Kevin, Alex, =ΡΡ= 419