WoW! The touch of the master. It has worked!
So what I have done:
1. download gimp from http://gimp.org/
2. download gmic from http://karo03.bplaced.de/gmic/git/Macgmic_macosx_64-Sierra.dmg
3. Because the gimp is the one from gimp.org, I used then gmic_gimp_ngtk to put it in the location:
The ... Gmic Gtk appear in my menu (gray) and when open a photo, it is fully accessible.
PS I am assuming that if i use the gimp from gimponosx I must use the gmic_gimp_gtk in the plugins location. But there I see only the gimp 2.8.18, not the 2.8.22.
From my point of view, case closed (for the moment).