noooby:
Everyone is spouting off about how everyone carries a GPS these days and how they are much more accurate, and they are, but they are not MANDATORY in most circumstances.
Unfortunately GNSS units still are subject to RAIM outages so whilst this is the case I do not think that CASA will allow it to replace the magnetic compass no matter how much more accurate GNSS units are.
Creampuff:
If you have a transponder with Mode C, you can confirm the accuracy of the reading/s at altitude.
For this to be the case you would have to be certain that your altitude encoder, which gets its information from the same static source as your altimeter, is reading correctly. If there is an issue with the static pressure system both your altimeter and altitude encoder will be in error and the mode C readout to ATC isn't going tell you that. Whether or not a big enough discrepancy would result from this issue to cause any real dramas is another story if your altimeter was within tolerance prior to departure.