Quote from DingerX " GPS is not designed to give altitude".
GPS is as accurate in altitude as it is in Lat/Long. The accuracy in any dimension is dependent on line of sight to as many satellites as possible in different directions and planes. In the air it is very accurate under most conditions in all dimensions provided it is connected to an external antenna. On the ground in wide open country it is also very accurate, while in towns or very mountainous areas it can become quite inaccurate in any or all dimensions. A phone GPS will not work properly inside a large aircraft as the body is sufficient to block most satellites unless they have line of sight through a window and it needs a minimum of 3 or 4 to obtain a lock with minimal accuracy. I fly gliders and use GPS for height readings and it is extremely accurate, much more so than a barometric altimeter!