Thanks to the help from the +GIMP
folks, I've been able to start the (big) work on the G'MIC plug-in to make it use the high-bits depth buffers (16bits and 32bits-float) provided by GEGL in GIMP 2.9
If everything goes well, you should be then able to run all G'MIC effects with full precision soon, directly from the GIMP plug-in (e.g. on 16bits and 32bits-float/channel images). That was of course already possible from the command line interface, but not from the current plug-in.
This is still work in progress anyway. Who knows what may happen ?
for taking time to teach me some GEGL basics :)