Without consulting a NMEA spec to see if a standard message exists with the data you are seeking, my guess would be the same as Pavel/nohal. Your GPSmap is calculating TURN and displaying. It may or may not output that on
NMEA. The NMEA standard costs money
to buy. Some reverse engineered information is available on the web such as NMEA data
I looks like some messages are defined that would have your information such as the BWC message. So the next step would be to find a reference manual for your gear
and see what sentences can be programmed on output.
You don't specifically mention OpenCPN
in your question, but if that is your intent, then you need to check against the messages that OpenCPN can use. Or as Pavel suggests, look for a similar function built into OpenCPN.
I suggest you clarify if you are interested in learning
about potential NMEA information from your unit, or (and?) if you are interested in potential use of said sentence in OpenCPn.