I've asked the G'MIC team today if they have a Max RGB filter (these guys are very responsive, truly fantastic) >
https://discuss.pixls.us/t/does-gmic-ha ... -rgb/25880They gave me very simple code to input (copy/paste) for now (may be "afre" will do a filter)
So in G'MIC go to "Various" > "Custom Code"
In the "Channels" drop-down menu select "RGB[All]"
then input (copy/past) that code:
For Maximum RGB
For Minimum RGB
EDIT:
For G'MIC 2.9.8 and above "afre" did made a filters > G'MIC > Testing > afre > Query Primary
There is even more choice than the original Maximum RGB from GIMP, and the afre's filter works like a charm