VDI combined with PI machines apparently aren't meant to be, or it would have been done by now.
BUT - you CAN accurately ID ferrous vs non-ferrous with a GPX, I do it all the time. With some practice, you can also predict the difference between brass (low conductor like a button) vs lead (mid-high conductor) a high percentage of the time - all through the audio response. It takes a lot of practice, but it can be done.
None of the other ground balancing PI detectors on the market can do this to the degree you can with the GPX, though. That's why a lot of guys prefer it in the relic fields as compared to the competition.
And there is still the limitation of very slow recovery times effecting a PI detector in general, which keeps you from using one in trashy areas.