I also agree with what @chris and @Iain said. A more permissive license would make things easier, I think 
I agree with Morgan. Time’s not free; looks like a good way to get some much needed fund to make G’MIC even better. Win/win for all. Just don’t let those potential vendors give you the shaft, David.
I support any change which you consider best for G’MIC core… would also be happy to change the license in my own community file. Whether I get credit or otherwise for anything I write is no concern to me, if somebody else finds it useful that’s good!
G’MIC next release (v.2.1.9), planed on next monday will integrate a change of license.
Some of the source code files will be dual-licensed CeCILL / CeCILL-C. This includes all the source code of the C++ libgmic, as well the implementation of the image processing filters and commands from the G’MIC stdlib.
I hope this will raise interest and contribute to get fruitful collaborations with developers around the world.