And I will reiterate, GPS cannot be used for navigation where positional accuracy is critical (terminal and approach phases)in a non-WGS-84 environment. In those flight phases, ground-based navaids must be used, i.e. DME/DME or VOR/DME RNAV or raw data unless using the correct datum.
There have been some real close calls with map shifts in the early days of GPS and WGS-84 involving terrain. Which is why ICAO made WGS-84 the international standard, it wasn't without reason. I have seen some colossal track and distance errors between what was "out the window" and what the EGPWS, using GPS-derived position, said.
GF