Because it's self-contained.
Because what is self-contained?
Do you mean that GPS is self-contained, and therefore unsuitable for RVSM? Do you mean that because GPS is suggested to be more accurate than a baro altimeter, that it's therefore unsuitable for reduced minimum separation airspace?
Which brand of barometric altimeter connects to the ADC?
Every one that's used in aircraft equipped with an ADC. It's one of the primary functions of the ADC.
I am not familiar with the true altitude terminology, btw. Is that a standard term in the States, or is it informal use to distinguish from barometric altitude?
You're not familiar with true altitude? It's a basic aeronautical term, not a US term.
True altitude is actual altitude. It is calibrated altitude, corrected for temperature.