It would be nice if G'MIC was integrated into Paintstorm. The new version had the license for the core (basically everything but UI) changed to CeCILL-C which means it can be used in closed source software. For more info read here: https://pixls.us/blog/2018/02/g-mic-2-2/
The only thing you can't use is the QT UI (just use the native UI stuff for that I guess). Here's instructions for how to use the core library: http://gmic.eu/libgmic.shtml
and the code itself: https://github.com/dtschump/gmic