There are no private servers for Gunbound.
Played the game before, for a loooooong time. Still have the account, but not playing anymore.
TheBeast:
http://www.softnyx.net/=>
Click game list => Gunbound.
It's a game that looks like worms, but you have more options here, you can buy avatars etc etc. Fun game to try, will get bored after 2 months.