To apply the law equally, the driver can't provide a certificate of calibration for his GPS device so legally he's unable to prove that his GPS is giving an accurate speed measurement.
(As an aside, I don't think GPS would require any calibration anyway. If I understand it correctly, it'll pretty much either work or not work, with the accuracy of the output determined largely by atmospheric conditions and satellite geometry. The worry here would be deliberate tampering rather than calibration.)
But I agree that by far the more significant problem would be deliberate alterations to the data. It doesn't seem like it would be particularly hard to do so...
You're probably better off just paying the ticket.
Minor traffic offenses are civil, not criminal, so the much lower preponderence standard applies.