Reason:

The GPS Altitude is more precise for final glide calculations since there is no temperature error (4% per 10 degrees deltaT from ISA-conditions)
The pressure altitude if corrected for the actual QNH however is the relevant Altitude for vertical airspace distances ASL. And the 1013 corrected pressure altitude for FL-airspaces respectively.
Competition scoring programs work with press-alt to check for airspace violations as well. As far as I know.

Suggestion for implementation:

If pressure Alt. is NOT available (or not desired):

Glide) use GPS Alt. for final glide calculations (will be precise for final glides)

ASL )use GPS Alt as ASL-Airspace altitude (there will an error since pressure alt would be correct to use)

1013)use the QHN from Settings Basic to calculate the difference between ASL-Airsp-Alt and 1013-Airsp-Alt (there will an error since pressure alt corrected to 1013 would be correct to use)

1013-Airsp-Alt = ASL-Airsp-Alt + (1013,25 hPa - QNH ) * bH

barometr. Höhenstufe (bH) = 8,3 m/hPa or 27 ft/hPa

If pressure Alt. and Gps Alt available:

Glide) use GPS Alt. for final glide calculations (will be precise for final glides)

ASL & 1013)
On the ground calibration:
Since the QNH shall be configurable in flight. (Which changes ASL-Airsp-Alt but doesn't affect 1013-Airsp-Alt) it is necessary to calibrate the NMEA-Pressure-Alt to 1013 first (i.e. calculate the delta between NMEA-Alt and Altitude above 1013):

• determine if on an Airfield, if yes use Airfield elevation (maybe crosscheck with GPS-Alt) if not use GPS-Alt to...
• calculate deltaAlt1 = AirfieldElevation? - NMEA-Press-Alt-ground

deltaAlt1 would be the value to correct NMEA-Alt to ASL-Airsp-Alt :
In flight: ASL-Airspace-Alt = NMEA-Press-Alt-flight + deltaAlt1

However we can't just use deltaAlt1, since changing the QNH in flight would have no effect on ASL-Airsp.Alt therefore:
1013-Airsp-Alt = ASL-Airsp-Alt + (1013,25 hPa - QNH) * bH (see above)
1013-Airsp-Alt = NMEA-Press-Alt-flight + deltaAlt1 + (1013,25 hPa - QNH) * bH

therefore: deltaAlt2 = 1013-Airsp-Alt - NMEA-Press-Alt-flight = deltaAlt1 + (1013,25 hPa - QNH) * bH

deltaAlt2 = AirfieldElevation? - NMEA-Press-Alt-ground + (1013,25 hPa - QNH) *bH

This calibration should be done only once on the ground and then deltaAlt2 should be saved for crashes and inflight restart. Provided the QNH to calculate it was correct, it is a constant depending only on the used pressure Alt measurement device.

1013-Airsp-Alt = NMEA-Press-Alt-flight + deltaAlt2

From here, using the set QNH, ASL-Airsp-Alt can be calculated:

ASL-Airsp-Alt = 1013-Airsp.Alt - (1013,25 hPa - QNH ) * bH

changing the QNH in flight will now have effect on ASL-Airsp-Alt and not on 1013-Airsp-Alt, which is correct.

This is imo the best setup for precise glide and correct airspace altitudes. It should be default.

However for personal taste of users maybe an expert-setting in the config:

"For glide calculations use:" GPS-Alt (default) / corrected Pressure Alt
"For vertical airspace distances use: corrected Pressure Alt(default) / GPS-Alt