I downloaded and successfully installed Gimp 2.6.6 and X11 1.1.1, upgraded X11 to version 1.1.3. After playing with GIMP a few times X11 behaves strangely. Now, I cannot quit. If I quit GIMP, all I have is a gray screen. I have to force quit X11 to get out. I can leave GIMP via on-line help, and quit X11 from the dock. If I launch X11 alone, I get a gray screen and have to force quit. So it seems that the problem is not with GIMP but with X11.
I am asking this question here, because I couldn't find an answer at Apple.com.
PS I am running OSX 10.4.11 on a G5 iMac