What is the Difference Between GNSS and GPS?

GNSS (or Global Navigation Satellite System) is a collective term for different types of satellite navigation systems used globally like GPS, GLONASS, Galileo and BeiDou etc. GNSS based systems can access multiple satellite systems at all times for better accuracy, redundancy and availability. 

While GPS (or Global Positioning System) is the Global Navigation Satellite System launched and owned by the United States. GPS based systems can only access satellites launched by the United States Department of Defense (DoD). GPS was originally built for military use, but was made available for civilian use in the 1980s. Even though GPS was created by the United States, it is used for various positioning applications all over the world. The other GNSS technologies are relatively new and will take some time for widespread adoption.