First get a route on the map using "Get Directions". Then use "Get GPX" to download a GPX file for GPS navigation.
Note: business names will not work for GPX; use a street address or lat,long position
August 2015 Changes to Google Maps and Google Earth mean that the map here may no longer match the downloaded GPX file. The GPX appears to match the direct Google Maps output, and may contain more than one alternate route for the same mode of transportation.
TRK (track) format will work as a route in Maemo Mapper. RTE (route) format will work as a route for some Garmin GPS sets via MapSource software. (may be broken)
The GPX conversion service http://gpx.geotags.com/cgi-bin/gpx.cgi?saddr=%s&daddr=%s should work for live route downloads in Maemo Mapper.
See also this page to find lat/long from a map or street address