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.
maciekmm1997, Jailsao321, Lamb0nfire, Jesus Bleach, bydunik, _Entity999YT_, LamGaming, 8GN8, Minecrafter9727, frangelCraft, beans, runasas, otman.ch, Twister, MaKuX, jdw6701, tondeknoi, bivey015, [T/Akuma]Scorpio