You will require special programs if you want to use these on your own PC or you can upload them to a web service for processing there.

One program that you may use is Garmins Mapsource.
The freeware program GPSBabel is quite suitable for converting the gdb files into other formats.

For information about GPS see: http://en.wikipedia.org/wiki/GPS
and for what a GPX file is: http://en.wikipedia.org/wiki/GPS_eXchange_Format