Leitura do GPS, atualmente uso GPS_PRIORIZE = 1

ESCRITA DO GPS.CPP

inline boolean newNMEAreceived() {
	return !resetingGPS && gps.newNMEAreceived();
}


void lerGPS(){
	if (!serialOff()) {
#if GPS_PRIORIZE > 2
		for (uint8_t i = 6; nmea_rb.ahead && nmea_rb.head != nmea_rb.tail && i > 0; --i) {
			// if the head isn't ahead of the tail, we don't have any characters
			volatile char* lastNMEA = nmea_rb.buffer[nmea_rb.tail];
			uint8_t oldtail = nmea_rb.tail;
			nmea_rb.tail = (uint8_t) (nmea_rb.tail + 1)
			% NMEA_BUFFER_SIZE;
			nmea_rb.swith &= nmea_rb.tail >oldtail;//se verdadeiro indica que não virou ainda.
			if(nmea_rb.swith)
			nmea_rb.ahead = nmea_rb.tail >= nmea_rb.head;
			else
			nmea_rb.ahead = nmea_rb.tail <= nmea_rb.head;
			Serial.println((char*) lastNMEA);// this also sets the newNMEAreceived() flag to false
		}
#endif
#if  GPS_PRIORIZE == 1
		if (newNMEAreceived()) {
			// a tricky thing here is if we print the NMEA sentence, or data
			// we end up not listening and catching other sentences!
			// so be very wary if using OUTPUT_ALLDATA and trytng to print out data
			char* lastNMEA = gps.lastNMEA();
			if (!serialOff()) {
				Serial.println(lastNMEA); // this also sets the newNMEAreceived() flag to false
			}

		} else {
			gpsNoNewNMEACount++;
			if (gpsNoNewNMEACount > GPS_NO_NMEA_COUNT_LIMIT
					&& (millis() - gpsNoNewNMEACountShowTime)
							> GPS_NO_NMEA_COUNT_SHOW_TIME_LIMIT
					&& !serialOff()) {
				Serial.print(PROTO_PREAMBULO);
				Serial.print("1110,");
				Serial.print((float) millis() / 1000);
				Serial.print(PROTO_SEPARADOR);
				Serial.print(gpsNoNewNMEACount);
				Serial.println("*");
				gpsNoNewNMEACountShowTime = millis();
			}
		}
#endif
	}
}

LEITURA DO GPS.CPP

// Interrupt is called once a millisecond,
// looks for any new GPS data, and stores it
ISR(TIMER0_COMPA_vect) {
	if (!resetingGPS) {
#if GPS_PRIORIZE > 10
		while (GPS_Serial.available()) {
#endif
		gps.read();
#if GPS_PRIORIZE > 2
		if (newNMEAreceived()) {
			uint8_t i = (uint8_t) (nmea_rb.head + 1) % NMEA_BUFFER_SIZE;

			// if we should be storing the received character into the location
			// just before the tail (meaning that the head would advance to the
			// current location of the tail), we're about to overflow the buffer
			// and so we don't write the character or advance the head.
			if (i != nmea_rb.tail) {
				nmea_rb.buffer[nmea_rb.head] = gps.lastNMEA();
				nmea_rb.head = i;
			}
		}
#endif
#if GPS_PRIORIZE > 10
	}
#endif
}