Well you can download the game from pretty much anywhere, it isn't hard to find.
But the official gunbound site is
here. You can download it from there (it's about 90MB) and it has tournament information, and everything else you'll need to know.
The thing about this game is that instead of getting a team of worms (or in this case avatars), you only get one and you can build up experience and money and buy new weapons. Or so I am told.