Independant of the hardware you're trying to use, when ever trying to interface a GPS, these are the things I would look at.
Serial Data Format: NMEA, TEXT, RTCM? (most of the time NMEA, some of the time, TEXT, I've never seen RTCM used in an application like this.)
Baud: 4800 (is pretty standard)
The electrical spec is most often RS-232 (+3 to +15, -3 to -15), but sometimes it can be TTL (0, +5). (Edit: is it a USB connection?)
And the map datum used: There are several. I think the most common for an application like this is "WGS 84".
I'd get out the manual for the radio and the manual for the GPS and make these settings match in both.
And if there are other common settings in the radio and the GPS, make them match also.