I want to express that ymb1's answer got my upvote, but that I have one reservation which I thought was worth putting here as an answer so it can be voted on and commented as appropriate.
I agree that the current tag gnss-gps should be renamed to gnss. However, I have a reservation about separate tags for the other systems - mainly that colloquially most people know the name "GPS" and will tag as such when what they really mean is the more generic "GNSS". I also do not think we will ever get enough questions about a specific system (such as "GLONASS") that would warrant having separate tags. And even if a question is about a specific system, tagging with gnss is still useful.
Therefore, my suggestion is as follows:
Note that earthscience.se seemed to agree with my point regarding the colloquial use of the term "GPS".