If the system was removed from the AD domain, it should remove all AD based GPOs. You can try running “GPUpdate /FORCE” to try to trigger it to recognize it is no longer in the domain.
Other settings could be from a local GPO setting, a local secpol template (unusual except in high security environments) or manually edited into the registry. All of this can lead to more than just basic GPO actions.
The difficult settings are custom GPO actions which tattoo the registry outside of the GPO registry keys. You will need to know what custom settings are used in your domain in order to revert them manually.
If all you need is to clobber the standard GPO settings while out of the domain delete all the GPO registry keys. This is a brute force method.
The four registry GPO roots: