I have the same problem stated here problem on 12.04 with gimp 2.8 but it is impossible to install ppa-purge, because of the fact that ubuntu software centre doesn't work anymore saying me I've to repair the dameged packages, but when I try to do it it doesn't work...like for Leonardo.... How can i do to repair the ubuntu software centre and repair Gimp? Thanks.
Here is a method that should NOT be tried unless nothing else works...
At first wait and see if you get any other answer here...only if you dont,use this method..
In this method,we are going to delete the status entry of GIMP 2.8(Thus bypassing dpkg)
Enter these commands in the terminal-
now enter the following command to open the status file as a superuser in gedit
In the file search for a block that will start with something like this:(you may use CLTR+F to find it.just put gimp in that search box.)
After you find it,select that complete block(Not even a single space or line more than that) and delete it.
Save the file and close it.
Then go in software sources and remove the repository.
You have successfully removed Gimp.
you can confirm it by typing in the terminal
You should get an error saying Package not found or No such package installed
NOTE1:FOLLOW ONLY THE GIVEN INSTRUCTIONS.NOTHING EXTRA IF YOU DO NOT WANT TO SCREW UP SOMETHING ELSE.DELETING EVEN ONE LINE EXTRA IN THE STATUS FILE WILL BREAK IT AND WILL NOT LET YOU UPDATE ANYTHING AFTER THAT(Though we have created a backup file its best to take care)
NOTE2:WHEN YOU OPEN THE STATUS FILE IN GEDIT,ALONG WITH IT YOU WILL FIND ANOTHER FILE OPENING UP IN THE BACKGROUND