I struggled with this very question too. Hopefully, given the title of your post, you will find the following helpful. If you already understand the difference, just go to the next post!
Before the advent of GPS, pilots used approach plates as directions for the approach and flew an approach by tuning in the initial fix on the VOR receiver, flying to it, then tuning the next fix and so on. There were no graphics and you learned to picture your position in your head.
A GPS receiver ?knows? where it is located (your present position) and virtually all have a database so it can present a map to show you your location and the location of nearby objects (roads, towns, airports etc).
A GPS navigator has the above functionality and an expanded database. Like other GPS receivers, with its database, it ?knows? the location of navigation points (VORs, intersections, runways, etc) which are commonly called waypoints. But its database also contains approach data. A requirement of GPS approaches is that the approach must be contained in the database and that there is automatic sequencing of the waypoints in an approach. So when you load and activate a GPS approach, the navigator will provide guidance to the initial fix and when you get there it will automatically sequence to each fix in the approach. This is why a GPS navigator is needed to fly GPS approaches.
A GPS receiver, even a WAAS receiver, does not have the database to provide automatic sequencing. WAAS adds more precision to the location information of a GPS receiver and that is why a WAAS GPS navigator is required for LPV approaches. The extra precision is needed for ADSB also.
Garmin revolutionized navigation with the GNS430 about 20 years ago. They combined a nav, com and GPS navigator in one box and they use the GPS to not only fly GPS procedures, but also to fly VOR and ILS procedures, switching to the VOR or ILS receiver for the segment from the final approach fix to the runway. This makes the setup and procedure for flying every kind of approach identical, simplifying workload.
My understanding from the install manual, is that a GPS175 has the capability to provide GPS location to an EFIS like your AFS- 5550. This will be a RS232 serial link. With this you do not need a separate GPS for your EFIS although you may want one for redundancy. It also will provide GPS ARINC data to your EFIS which provides longitudinal and vertical steering. It also has the Garmin propriety GPS information for Garmin ADSB out solutions.
Typically you will control your SV autopilot from your AFS-550. However most installations usually provide a redundant mode to control an autopilot directly. I think this is done with the Dynon autopilot control panel and it will use the RS232 GPS data as well as the GPS ARINC data.
Jim Butcher