Just saw that this cooperative action game is on GitHub (always has been, even, but I didn't use GitHub then). It was an entry for the first RubyWeekend competition, and since the respective forum has been lost, I thought I'd put the link here:

http://github.com/joakimk/caribbean-onslaught/tree/master

To my knowledge it's the only Gosu game with cooperative gameplay for up to three players :)