Do you actually need to escape the GPL? All the GPL requires you to do is release your own source code under the GPL. It does not apply to game levels, art assets etc. You can still sell your game and make the code available via the GPL to people who buy it if they want it, or make it available online to everyone.
The only issue is if you want to use third-party non-GPL libraries, as then you won't be able to distribute them along with your GPL engine.
If you do need to escape the GPL, look at their licensing page they sound flexible enough that you should be able to come to an agreement.