bose-x
Are you 100% sure that the altitude displayed on a GNSx30 / KLN94B is anything to do with the altitude encoder connected to the GPS?
As I posted higher above, the encoder is a RAIM requirement, and should generate a RAIM warning if there is something like a 200ft diff between the GPS solution altitude and the output of the encoder.
For this to work the encoder needs to know the QNH, which is why the QNH has to be entered.
My GPS ('94B) doesn't display the GPS altitude unless it gets a signal from enough satellites, more than 3 I think, and even then it fluctuates by say 50ft until it picks up more of them. If it was derived from the altitude encoder it would not do that.