I found an interesting website:
http://www.ertico.com/links/gdf/gdf.htm

Here you find the specifications on the GDF map data format which is used by Teleatlas together with some sample files and explanations.

Additionally I found a Linux project on car navigation and some interesting links on this site:
http://www.geocities.com/linuxcar/GPS_car_Nav_EN.html

Probably some code of this project can be reused for the gpsapp.

Juergen