GPS plugs into the rear serial DB9 port. From the manual:
The GPS device must have a serial (RS-232) output, and be capable of outputting standard NMEA-0183 v3.01 compliant location data, and GGA/RMC data sentences at 4800 bps. Make sure that the cable terminates in a female, 9-pin serial connector and you select the correct baud rate (4800 bps) for the rear serial port.
The port can be set for other baud rates than just 4800. The new Uniden GPS module uses 9600.