I'm not involved with GIMP except for doing translations, so I'm just spreading the information. From a casual view contributing to GIMP has been made much easier/modern in the recent year or so, and they've worked to document and visualize things a lot. GIMP needs contributions, it's very under-resourced for the importance it has and compared to some others (like Blender for example), and probably the reasons have been the long development history and culture from there. But now those seem resolved. GIMP has become much more "fresh", the 2.8 is finally out, GEGL has indeed gotten up to speed so hopefully new developers will join!