It's not just NMEA 0183 vs NMEA 2000, it is their weird choice to have two output ports on the
radio, one of which sends different sentences then the other. They "fixed" that on the GX2150 if you configure it at 38400. If you configure it at 4800 then one port talks 4800 and the other talks 38400, and they each send different sentences.
To work with your bluetooth adapter (which only has one serial input
line) you'd need to configure the
radio at 38400. If you configure it for 4800 then you won't get the AIS sentences, only the DSC ones. This is on page 16 of the manual.
Their NMEA 0183 implementation is weird in many respects. That doesn't mean that the device doesn't work, it does when properly interfaced. They just made it harder than necessary to interface with it. I'm glad that they invented the market segment, and I'm glad that other companies are coming out with better options in this market segment.