Removing GPO - comprehensive list of 'tattooed and non tattooed' registry settings
Solution 1:
The Group Policy editor will only show settings that do not "tattoo", by default (that is, entries that are made in either HKLM\Software\Policies, HKLM\Software\Microsoft\Windows\CurrentVersion\Policies, or the HKCU versions of the same). You have to alter the filter to display settings that are not "Managed" to see entries that are made outside those portions of the registry.
Settings made within those keys are removed when the policy no longer applies. Settings outside those keys will "tattoo" the registry.