This board consists on a gps connected to a stm32 using the serial port (uart), and a magnetometer (hmc5883L). We convert the nmea data from the GPS to an i2c binary protocol in a similar way that the i2c-gps-nav project. The magnetometer is just included in the board, connected to the same i2c bus.
Why? The idea is to use this board in a multicopter. Having the magnetometer far away from everything else makes sense (magnetic interference from motors/battery/motor drives). Magnetic interference is specially important in big multicopters. We include the gps, because it is also usefull to have it well exposed away from everything else, specially other transmitting sources (telemetry transmitter, FPV video TX).