This is a not very accurate port of the game realtimebattle which I is written in C++ and I couldn't compile on OSX.
The basic principle: you program little bots and then they fight each other in an arena. A bot is an executable than receives information via STDIN and sends back answers via STDOUT. For an example see the bos/stupid_bot.rb. Bots can be written in any programming language, not just Ruby.
The GUI is implemented as a web application using Sinatra. You can start the server from the root level of the project directory like this:
$ ruby server/realtimebattleserver.rb