About altitude mode meaning

px4.io expressed altitude mode is non-gps, does the meaning of non-gps mean that other Glonass, Baidu, and Galileo among GNSS receive only GPS? Or it`s mean do not receive the entire GNSS?

It would mean non-GNSS. We usually use GPS and GNSS interchangeably. It’s up to the GNSS receiver what it supports, e.g. the uBlox module.