What are the differences between a GPS coupled autopilot and a GPSS autopilot?
Pugz provided some very good information back on September 29th.  As I recently installed a Garmin GTN 650 and add S-TEC GPSS to my 35 year old S-TEC 60-2 autopilot I thought I might respond also.  As I did this installation myself I am quite familiar with the wiring involved which helps in understanding how the autopilot responds so differently to GPSS (GPS Roll Steering) than it does to the CDI (Course Deviation Indicator).  First we need to understand that after installing GPSS the autopilot has two completely different interfaces.  When in NAV mode the autopilot responds to the CDI for lateral guidance (and the CDI glide slope if the autopilot has that capability like my 60-2 does.)  When in HDG mode the autopilot would normally respond to the Heading Bug on the Directional Gyro but if GPSS is installed the autopilot actually has two separate functions while in HDG mode:  1) Follow the HDG bug on the DG; 2) Follow the GPS Roll Steering commands that are provided by the GPS via ARINC 429 output that is routed to the autopilot GPSS module and converted into Heading Commands as if the HDG bug were providing them.  In order to implement this the GPSS module has a switch to change from HDG mode to GPSS mode.  As far as the autopilot is concerned he is just getting heading commands from a HDG bug on the directional gyro.  The autopilot has no idea that he is being driven buy the GPS while in GPSS mode.  Just imagine that a very skilled pilot is operating the HGD bug on the DG and the pilot knows exactly when and how much heading change to make in order to turn smoothly on the next route segment or intercept that route segment if off course. This is exactly what is happening except that the very skilled pilot happens to be the GPS receiver.  The GPS knows exactly what heading changes need to be made and when to make them in order to guide the aircraft around a procedure turn and intercept the final approach course.  

The new Garmin GI-106B CDI that I installed along with the GTN 650 still has some very important functions.  After the GPS has guided the autopilot (via GPSS) through the entire route and possibly a procedure turn course reversal and intercepted the final approach course I switch the autopilot from HDG mode to NAV mode in order to engage the autopilot on the CDI.  Now if I am flying an ILS or RNAV LPV approach the autopilot has both vertical and lateral guidance via the CDI.  The CDI will either be getting input from the ILS signal or from the GPS depending on what has has been selected on the GTN 650.  Both autopilot interfaces are still important (HDG/GPSS &amp; CDI) but the autopilot CDI interface is used only for the final approach where vertical guidance is needed.  Most of flight is best flown with the autopilot engaged in HDG mode either flying on the actual heading bug (being vectored) or with GPSS engaged and the GPS is turning the virtual heading bug for you in order to fly the programmed route.