Mostly there are stupid answers instead of stupid questions.
- If it uses serial, it is most likely NMEA0183 format. And you need to know right baudrate that is using. Note also that serial is a bit wide definition and hopefully you also know the physical output, since it can be TTL, RS422 or RS232. Connecting RS232 to TTL may burn TTL side.
NMEA0183ToN2k does a lot and it seem to already have basic GPS conversion. There is no direct way to do that. NMEA0183ToN2k reads data from NMEA0183 and collects it to variables (see BoatData.h). When it has enough data, it will forward it. It seem to send at least GNSS, SetN2kCOGSOGRapid and Heading messages.
- GNSS will be normally sent with 1 sec period. Heading and COGSOGRapid will be send with 100 ms or 50 ms period. NMEA0183ToN2k sends all messages as they arrive, since there is no data send faster.