You are correct to assume the GP150 has no connection to the Navnet 3D system. I just made a major update to my system and this is how I know.
The GP 150 only feeds the VHF and in your case the SSB. I have heard of cases where the output driver of the NEMA0183 output failed. As Bill said the GP 150 is no longer available and if it were, it is very expensive as it is a SOLAS certified GPS.
However there is some good news as the GP150 has three outputs for GPS data and at least one of them is still unused in the default Amel installation. You may be able to use this output as alternative. I know this because I used that output to feed the EPIRB I have on the boat (Some fancy ACR thing).
You may need to alter the config of the GP150 to get the output to work in the way required.
For your information I also made a connection between the fast-heading output of the NavPilot 500 to the input NMEA input of the B&G H3000 CPU. On my boat Amel forgot to do that so the H3000 CPU had no way to calculate the ground wind (wind direction relative to the north). This enable you to plot a graph on the H3000 graphic display to see the wind direction changes over time.
Let me know if you need more information.