Like Alexban, I also am not aware of any 'GPS fits' in Classics other than a GPS card which can be fitted for EGPWS, but has no input to navigation.
Still, there are miriad versions of 737's around, so who knows (Captain Sandl?) but I suspect you have a 'standard' nav kit with a rogue IRS. Line selecting to the 'good' IRS would ease the problem.
If you do have GPS navigation it may be a 'patch' installation and the system may not default to the usual degradation rules?