ADS-B format retains both ways of reporting barometric as well as gps altitude
Yes but the ICAO standard chosen was Mode S/Baro, not the GPS Alt. No single a/c ADS equipped flying today uses and transmit GPS Alt.
ATCWatcher, Pls recheck the docs:
DF17 velocity squitter (subtype 19) does transmit the difference between baro altitude and GNSS height.
A simple addition then calculates the GNSS height.
All newer transponders do transmit this and I can see it if I want (though it do not know what to do with it)