GNSS and clock comparisons measure different things, one is a based on radio wave travel time between a receiver and the satellites in the constellation, the ot

GNSS and clock comparisons measure different things, one is a based on radio wave travel time between a receiver and the satellites in the constellation, the other is a physical measurement. That's what I was referring to wrt independence: a gravity measurement would avoid any potential error source in the whole chain of the GNSS system (which is quite lengthy). The geoid has been partially satellite-derived for some time, see the GRAIL, GRACE, and GRACE-FO missions. The US is also running a nationwide airborne gravity program at the moment, too, which an impressive undertaking and should be done soon-ish. Comparing a measurement of actual local gravity against the geoid-stated gravity should be enough to give the elevation relative to the geoid, which is the same output that you get from GNSS. That's my intuition, anyway.