I sent an email to ICOM and received the following reply:
"The way the radio is populated via the GPS is that the GPS unit you would be using would have the Jeppersen's database loaded on it (usually via a SD Card). The GPS unit would send that data along with the GPS data to the radio. The radio in turn would populate the GPS memories/standby memory in a rolling fashion during your flight. Pins 7 and 8 would be your GPS connection. You will have to use a GPS unit that has a NMEA 0183 output with a connection speed of 4800 baud."
The ICOM A-220 installation manual indicates for the rear connector, pin 6 is ground, pin 7 is serial data OUT and pin 8 is serial data IN.
I then called ICOM tech support and asked for further clarification on the NMEA sentence string structure and was told it uses NMEA-FSI.
Does this help?