For reference, aircraft explicitly DO NOT use GPS for altitude. Aircraft altitude is always barometric primary. An ILS approach can utilize GPS but the primary altitude data will be from a barometer.
A standard GPS unit has a vertical accuracy of about 10m vs about 3m for lat/long. Barometric altitude is more sensitive for small changes in elevation, but the base elevation can vary based on the overall weather. For aircraft, take-off and landing, altitude is based on a FAA-certified barometer at the airport. Above 18k ft all aircraft are flying at pressure altitude of 29.92inHg, the actual altitude will vary according weather, but the important part is that all aircraft vary TOGETHER. The most important thing is that aircraft maintain 1000ft vertical separation.
For a bicycle, barometric altitude will give you a better estimate of elevation gain & loss, but potentially less accurate for the absolute altitude, such as the top of a hill.

A Garmin unit will use wheel speed sensors for speed and distance, but it calibrates that number very accurately using GPS. The traditional method of wheel size calibration is using a roll-out test and a tape measure. The Garmin unit does the same thing, but all it needs is basically a 1km long section of straight road.

The next generation of GPS sensors that should be hitting the market relatively soon will have ~cm level accuracy. Those sensors are based on using both L1 & L5 GPS bands, and carrier phase tracking. This technology already exists in survey grade GPS units and that's why the performance isn't comparable to a bicycle unit. Autonomous vehicles really need cm level GPS accuracy to enable lane tracking.