04-07-2020, 08:29 PM
Uncheck the ppa for gimp then sudo apt-get update. Here is step by step. Go to the menu and find software and updates and click that then go to other software then uncheck that ppa, then it should reload the lists, it is a good idea to run sudo apt-get updates to make sure it was fixed. The fix is to edit it and add stable after it as the error showed you but unchecking the ppa will get you back up and running until you can fix the issue which should have a thread on the forums.