GPS technically only refers to the system established by the US, whereas GNSS is a general term including Beidou, Galileo, and GLONASS. (any others?).
While GNSS is the more general term, many users may still search for a GPS tag because the term has been around so long it's used (incorrectly) interchangeably with GNSS in popular context.
I've made an edit to update the gnss Wiki excerpt which is currently pending.
Global Navigation Satellite System, including GPS, GLONASS, Galileo and BeiDou
Question: Would it make sense to use gnss as the tag for all related questions, and convert GPS to a synonym? If so, what would be required?