The only GPS which can be legally used as your primary navigation source for IFR operations must:

1) have a current database.
2) be permanently installed with Mx paperwork (vice handheld/dashtop).
3) FAA approved for IFR use.


However, for any VFR or IFR ops it is legal (and prudent) to use any and all means of navigation for backup and situational awareness...this includes a non-certified or non-current GPS. The catch is that you must use other nav means for legally required navigation performance (ie IFR), and must use the non-GPS nav as your primary nav system(s).

The GPS-only approach presents a special problem here...since it probably has GPS-only waypoints, you cannot use any other nav system as primary, so you cannot do GPS approaches. An exception would be a GPS/RNAV aproach if you have an FMS with DME/DME capability and the specified RNP...this you could do with the FMS and no GPS (I have done these while working for an airline which was too cheap to buy GPS).

If you are doing flight training, you can do VFR practice approaches with an expired database, just be wary about accepting "direct to" routing from ATC to a GPS-only waypoint. Even VFR, accepting such routing would imply that you have certified and current GPS or RNAV equipment on board.