All GPS receivers transmit their data using the NMEA message protocol which, from the processor view, is a standard modem interface 48Kbps, 8 bits, no parity, 1 stop bit. Some newer GPS devices (such a Origin) also output data on SPI and/or I2C but still retain the standard UART so that they are always backwards compatible. You can find some useful information about NMEA formats at NMEA data.