This is the confusion we have generated in the last 20 years by using the FMGS to fly non-precision approaches.
To reiterate: If you are flying a conventional non-precision approach i.e. LOC, VOR or NDB, then the navaid must be tuned, identified and displayed at all times and it is your primary reference.
We can and do use the FMGS for guidance, meaning using it to steer the airplane, but at all times the primary reference is the navaid stated in the procedure you are flying. That is a regulatory requirement everywhere in the world and there is no relief from that. If the raw data shows you going off track, then you have to do something different to bring it back. Nowhere do you reference RNP because your reference is the navaid.
If your only reference is the FMGC, then you are doing an RNAV approach and they all, without exception, have an RNP attached. Usually it is 0.3 for basic RNAV, but there are RNAV AR and RNP approaches that are more stringent. Conventional navaids are not part of the RNAV picture in any way with the exception of critical DME's in the case of RNAV 1 SID's and STAR's if your aircraft is not GPS equipped.
I can't make it any simpler than that. If you are doing an ILS, VOR or NDB the navaid is your primary reference and has to be there by regulation. The use of FMGC's for guidance in that case is a technique, but doesn't negate the requirement for the navaid.