Hi, training wheels. This is an excerpt from a typical FMS setup for a modern airliner:
WITHOUT GPS PRIMARY
The FMS position is computed from the three IRS positions, that are combined to provide a MIXIRS position. The radio position is also combined, if two DMEs, a VOR/DME or a GPS supplemental are available. The GPS supplemental is considered to be an additional form of NAVAID, and can be accepted, if it falls within the radio position or the MIXIRS position.
WITH GPS PRIMARY
The GPS interfaces directly with the IRS that outputs a GPIRS position. When a GPIRS position is available, it overrides the RADIO position, if available. Therefore,the FMS position tends toward the GPIRS position.
GPS is Primary if GPS is used as the primary source of navigation by the FMS. This happens on long oceanic flights where ground navaids are not available. If GPS is unavailable for whatever reason, the FMS will display a GPS downgrade message and you will have to rely on available navaid signals to quantify the position error of the FMS. In this case you should not rely on the FMS data for an approach and you would probably seek vectors and fly raw data based on ground navaids for greater navigation accuracy.