You problem stems from the upgrade from 11.04 to 11.10. It appears you have
also held broken packages, and you most likely need to fix those. It should
be under SYNAPTICS -> EDIT -> FIX BROKEN PACKAGES
To get around this you should look to see if gimp-data is installed, and if it is,
remove it as it is an older version ((<= 2.6.11-z) but 2.7.3-2011052002).
Then you can just open a terminal and install gimp-data.
which should get the dependency installed. Then just install gimp.
sudo apt-get install gimp-data --force-yes
That should work.......
sudo apt-get install gimp