now gps nmea library is support GPS mesages only

GP is Talker ID (GP for a GPS unit, GN for a GLONASS)
Mediatek 3333 & ublox lea 6 h support glonass system 
as example mt 3333 after gps and glonass fixed output mesages changes 
from $GPRMC to $GNRMC and APM show "NO FIX" 


// NMEA message identifiers //////////////////////////////////////////////////// // 

const char AP_GPS_NMEA::_gprmc_string[] PROGMEM = "GPRMC"; 
const char AP_GPS_NMEA::_gpgga_string[] PROGMEM = "GPGGA";
const char AP_GPS_NMEA::_gpvtg_string[] PROGMEM = "GPVTG";   

... 

if (!strcmp_P(_term, _gprmc_string)) {             _sentence_type = _GPS_SENTENCE_GPRMC;         } 
else if (!strcmp_P(_term, _gpgga_string)) {             _sentence_type = _GPS_SENTENCE_GPGGA;  }
else if (!strcmp_P(_term, _gpvtg_string)) {             _sentence_type = _GPS_SENTENCE_GPVTG;}

to increase the number of compatible receivers
I propose to make such changes:

const char AP_GPS_NMEA::_gprmc_string[] PROGMEM = "RMC"; 
const char AP_GPS_NMEA::_gpgga_string[] PROGMEM = "GGA";
const char AP_GPS_NMEA::_gpvtg_string[] PROGMEM = "VTG"; 

if (!strcmp_P(_term + 2, _gprmc_string)) {             _sentence_type = _GPS_SENTENCE_GPRMC;         } 
else if (!strcmp_P(_term + 2, _gpgga_string)) {             _sentence_type = _GPS_SENTENCE_GPGGA;  }
else if (!strcmp_P(_term + 2, _gpvtg_string)) {             _sentence_type = _GPS_SENTENCE_GPVTG;}