GPS is usually not used for altitude.
Transition altitude (ascent) and transition level (descent) is typically defined per FIR (flight information region), but can be airport-dependent. It is listed in the relevant airport charts. 7000 is fairly common, but it varies. Altimeter setting is measured in hPa/millibars (with the standard being 1013.25, which corresponds to 29.92 inHg).
The most important thing is that everyone in an area reference the same altitude/altimeter setting for traffic avoidance. Whether or not that is off from the actual altitude by a few hundred feet or not is secondary, since ground avoidance typically uses a lot bigger safety margins. Airliners and military planes have radar altimeters to measure height above ground when flying in IMC (low vis), while smaller planes typically fly VFR (based on visual rather than instruments).
But the RA are used primarily in the later phases of the approach. The barometric altimeter setting remains vitally important throughout most IFR ops in all categories of aircraft. (Radar altimeters are a key component of the ground proximity warning system GPWS in transport aircraft.)
> while smaller planes typically fly VFR (based on visual rather than instruments).
General aviation aircraft are quite frequently well-equipped with instrumentation for IFR ops. I’ve flown approaches down to published minima in single engine aircraft and have (legally) departed in low IMC conditions that grounded Part 121 flights. (Not necessarily best judgement…but here to tell the tale that I’ve become wiser in older age…)
Indeed, the context of my sentence was using it for ground avoidance.
> General aviation aircraft are quite frequently well-equipped with instrumentation for IFR ops.
Yes, but most GA flights are VFR flights. Again, the context was ground avoidance and my point was that since they are usually used for VFR, more advanced means of ground avoidance are not generally required.