That looks interesting and well done @Philemo.
May I ask if the sources of the plug-in are available or not (or will be available some day?).
The CeCILL license has a ‘contaminating’ clause (just as the GPL), which means that linking a binary with the G’MIC library forces the sources of the binary to be opened as well. Is that your case ? Or don’t you have direct linking to the G’MIC library ?