gps works without 3g / edge. it uses assisted gps, and those towers "help" the gps out.
you can still use a program like navigon or tomtom where the maps are saved on the device itself. the things that would not be available are traffic and a local google search. say if you know the name of a place but not the address. if you had an internet connection, it would search for it and you could "navigate to" wherever it was. without that internet connection, you can only use data that has been saved in the program or device.