Difficult question. My understanding at the time (90s) was that after a certain MSN, all MMRs fitted included GPS which the operator had to pay extra to have activated. Presumably ac prior to that MSN could be retro-fitted with the GPS capable MMRs.
My airline at the time had 320s which were not GPS capable, but the GPWS box fitted was. So the GPWS knew where we were even if we didn't!