Blogs,
GPS was replaced by GNSS simply because the term GPS was US centric, the NAVSTAR Global Positioning System (GPS), which originally was military based.
There was a different system put up by the Russians, GLONASS, but it used FDMA, and was not compatible with the US GPS CDMA signal, but has recently been switching over.
ICAO had to do something different by calling it the Global Navigation Satellite System.
The difference with GNSS, for example, with the EU Galileo system, is that this provides full integrity signal in a civilian system. It does operate on different frequencies than GPS, and receivers are designed to combine GPA and GNSS signals.