There are navigation satellites launched by different countries and USA was the first one to do so.
Here are some major players in the navigation space,
Global Positioning System - GPS :
The satellite constellation launched by USA covered the whole world and it was called the NAVSTAR and later as Global Positioning System abbreviated as GPS. The more the number of satellites, the more area they can cover.
GPS of USA has the biggest coverage all over the world so everyone uses GPS satellites for navigation.
Russian GLONASS :
Next , A contemporary satellite constellation was launched by Russia called GLONASS. It didn't have full global coverage like GPS but has improved in 2000's.
European Galileo :
European union launched their own satellite constellation called GALILEO and it serves the European union.
Chinese BEIDOU :
China launched their own satellite constellation system called BEIDOU.
Indian NAVIC :
India is launching indigenous satellite constellation called NAVIC/GAGAN. It will serve the Indian sub continent.
What is GPS module ?
GPS module is the one which only supports navigation using USA satellite system.
What is a GNSS module ?
GNSS module is the one which supports all major global satellite navigation systems including GPS, GLONASS, NAVIC, BEIDOU and GALILEO.
Why would each country launch their own Navigation system ?
Each country wants to be self reliant so that in times of a war, the other country like USA might deny access to GPS signals or scramble them so that only their missiles and jets can make use of the GPS signals. So, it would be foolish to rely on GPS signals from USA satellites all the time.
If a country has its own satellite system then their ships, submarines, jets or missiles can utilize them securely.
Under normal circumstances using GPS signals for domestic purposes is totally fine.
Should you use GNSS or GPS module in your design ?
You should use a GPS module, If your application of your design is totally domestic and you have GPS coverage in the usage area.
You should use a GNSS module, If your designs are going to be used globally. GPS signals are not available everywhere. If your design has GNSS capabilities then it can connect to local satellite signals like NAVIC in India and Beidou in China for stronger GPS signal reception and better accuracy.
Cost factor :
GNSS modules cost more compared to GPS modules because of extra hardware added to support different constellations and frequencies.