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.
asdfgh_, The Gman, Doom Marine, petilo.ua, nikhilaggarwal420, ᶨᵘᵍᵍₑᵣᵑₐᵤᵗ92, [S2S] [Miccraft] tnt996, Alex, Nisse, Darkwolf, TheBananaThief