Hi, Photo Reactor is a Mediachance tool that allow to create filters through nodal processing (a sort of flow). Some free filters were free and created by users and were very interesting, downlodable inside their forum. The filters if a I remember well were embedded in images. If someone has still access to forum or to those filters, this could be used as inspiration to port them into G’MIC, obviously not by nodal processing but adapting them to gmic engine.
An old thread that might be insightful. 
G’MIC as nodebox nodes plugin - Software / G’MIC - discuss.pixls.us