I suggest that you try and send the AS-HHGPS back as you don’t need it to get the NMEA data from your 998c Si unit to the VHF radio. All you need to do is use the wires at the end of the second cable that comes out of the GPS Receiver connector. There will be either a Red, White and Black wire or a Green, White and Black wire. The important wires are the White (NMEA Output) and Black (NMEA Ground) wires. Be sure and tape off and secure the third wire so that it does not short to anything. I believe that you wiring should be:
VHF Radio wires - Humminbird GPS Receiver wires
Blue (GPS Input +) - White (GPS Output)
Green (GPS Input -) - Black (NMEA Ground)
Since the 998c Si unit does not allow an input from a DSC radio the Gray and Brown wires from your radio will not be used.
FYI: The As-HHGPS would be used for an AIS communications connection.