the true altitude displayed by the GPS (give or take some smallish errors) and the pressure altitude are the same if the QNH is 1013 AND the temperature profile of the air up to FL410, in this instance, is standard. otherwise they're not the same (which only happens all the time)
someone using a GPS in an airliner wouldnt bother me unless he was strapping a parachute on at the time or popping a full clip in his uzi or something