Completely remove package (GIMP), including all settings

Try deleting ~/.gimp. If you can't find that, it might be called something else, like ~/.gimp-2.6.

Note: Newer versions of GIMP may differ; for example, GIMP 2.10 might be in ~/.config/GIMP/2.10.

Since this question resurfaced, try in that order:

• Edit > Preferences > Windows management -> Reset saved window positions do default values (other Preference tabs also have a button to reset settings to defaults: Tool options, Interface, Input devices)
• In the Gimp profile, erase the sessionrc file (which is where windows positions are remembered)
• Rename the Gimp profile (so Gimp recreates one, and you can copy useful things form the old to the new)
• Reinstall Gimp (but never had to go that far)