"Class: Toybox::Position::Info" altitude seems to report on the barometric altitude. Explicit information on that could not be found, but looking at the reported values suggests that strongly.
Is there, and if yes, how, a way to obtain the "hard" GPS Altitude? Intention is to implement something like an InFlight-App.
Beyond that: The overall impression is, that the F3 UI-wise and SDK-wise hides (or does not expose) all the technical "atrocities" from(to) the user. Another evidence: Satellite View gone from UI, SDK abstracts any technical information down to (just) five "Position.QUALITY_*" values. Coming from a Fenix 1 and a Tactix (98% a Fenix 2), the ability to program the F3 is a outstanding and fantastic asset - but hopes to compensate the differences between those watches by adding own technical functionallity (data fields, apps) are slowly fainting away. The very nature of this watch series has not been upgraded, but rather changed radically. No complaints, just finding out the hard way that non-triathletic-tekkies, car drivers, flight monitoring junkies and the like don't seem to be anymore the target audience of neither this (wonderful) watch nor, for worse, the (cute) SDK. Until being teached otherwise :-) ...