I have the same problem here.
Not only gimp isn't working, beryl-manager doesn't take the right GTK+ either.
Because I have too much time, I purged KDE, X.org, all libglib2, all GTKs and so on. Was a nice session ;-)
After reinstalling KDE, x.org, gimp and so on, it complains the wrong version again.
There is an libgtk-x11-2.0.so in /usr/lib and in /usr/local/lib
ldd $(which gimp) | grep gtk
I found out, that gimp is depending on the one in /usr/local/lib, but the one in /usr/lib is the file from the package libgtk2.0-0.
apt-file finds no information about the file in /usr/local/lib.
Do you have any idea, how to solve this?