First off, I did not make this map! I found it a long time ago in a CSS server and found out it works well with gmod 10 as well. Don't give me any credit because all I did was upload it. Works great in death match games and Halo styled games.
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