That doesnt make sense. Unless you have a stand alone unit, the GPS input is through the IRU.
OK, I'll rephrase that for the pedants: Is the navigational data available to your FMS of an accuracy comparable to a GPS source, or does it originate from an inertial-based system, thereby being both less accurate and subject to drift ?
The object of the question being to establish whether the data that the OP downloads from his aircraft is of a sufficient accuracy to challenge, if necessary, the authorities' track-keeping data, which will be radar-based.