I would expect that the limitations on linking that at least the GPL imposes (and I think also CeCILL) are fairly counter productive and prevent what at least I would call good use of G’MIC. Then again, it makes selling licenses easier.
So I guess the questions is do you want to try to make money licensing the code, or offering consulting, development and other services around it.
As a user I would certainly appreciate a more permissive license.