Wolfgang Schwanke wrote:

In theory you could use GPS altitudes when flying MSL/QNH setting,

No. Even with MSL/QNH setting, as you call it, the barometric altimeter
does *not* show the true geometric altitude. More precisely, it only
shows it in two cases:

1. When sitting on the ground at the reference altitude for the given
altimeter setting (QNH)

2. When the atmosphere happens to be an ICAO standard atmosphere.