Just a quick update, I have unlocked online multiplayer mode in Fistful of Gun, if you see any bugs (which I am expecting quite a few!) please post them here:
http://steamcommunity.com/app/229810/discussions/0/613957600540389155/
Right now there is just one server operating but I will be releasing the server.exe shortly if you wish to host your own. Just update the game to the latest version on steam and you will be able to play it under the "online" option on the main menu.
Multiplayer also supports multiple players from each computer, so if you have 2 people on one computer, and 5 on another computer, you can connect and play 7 player together, not just limited to 1 person per computer :)
Enjoy!
Paul