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.
celly, TheOnlySpy ツ, |-GTC-| Rob, bjfmonkey, Connor, Flux, Kyco4ekMyaska, TCIAL22, <Axe's axe!>, MrSampart1, [SS]BlueSn00w, Main Fighter, Musteiz, The Big Boss of Taco Bell, Willymont, ETA-22, Blu Nautilus