I don't know what is going, but the high level symptom is 100% reproducible since my very first days on Max OS X: After not having used Gimp for about 1-2 months is will never ever start when I need it.
Today it even would not start when re-installing it. Not a big surprise since this was already the latest and greatest version.
What helped, however, was hard killing the X11 app: Twice.
After killing the X11 app it came back again and I killed that again. I assume the automatic starting of X11 by Gimp is broken or that some other X11 app leaves the X11 server in a state such that Gimp cannot deal with it.