Tim,
it is not the "GPS" that causes the offset. Some aircraft still feed "INS" data to their tranponders, those data beeing more or less incorrect, as the INS suffers (random) "DRIFT" (as a function of time) and requires (manual) updating from time to time by the flight crew, commonly performed if the aircraft "rests" at a stand between flights. Drift so NOT to be a "CONSTANT" that might be compensated for on your RB's display.
GPS data instead, if fed to the transponder (s), is what you experience if an aircraft is in coincidene with what you expect it to be regarding its position.
Rod,
ADS-B is not yet in use officially until today. Should it become whenever, and even be part of collision avoidance, it will of course rely on accurate GPS data without any doubt.
Kind regards
Karl