Which is funny, because I've heard it both ways.
For a while, as new systems were coming online, GPS was being used as a generic term, and the US system was being referred to by its original name, NAVSTAR.
But only purists would do that, and everyone else kept saying GPS to refer to the US system, so the new term GNSS was invented as the generic.
I try to say NAVSTAR and GNSS to avoid the ambiguous GPS, just like I say "gridiron" and "soccer" to avoid "football".