GNSS – Global Navigation Satellite System, a location provider based on satellites. GPS is the name of the USA satellites system. For many years it was the only one available for consumer devices so the term GPS is more common than GNSS.
Nowadays, there are several networks including the European Galileo network and Russian GLONASS network. As developers, the specific network used doesn’t really matter.
The result is the same, we get an accurate position.
Do note however that devices cannot receive the satellites signals indoors. You must test outdoors.
In B4A, you will use the GPS library to access location information based on GNSS. In most cases it is preferable to use FusedLocationProvider which will allow you to combine GNSS with other location providers that are based on wifi or cellular.
In B4i, you will use iLocation library which is similar to FusedLocationProvider and is not limited to a GNSS provider.
B4A FusedLocationProvider example: https://www.b4x.com/android/forum/threads/fusedlocationprovider-resolution-dialog.111652/#content
B4A GPS example: https://www.b4x.com/android/forum/threads/gps-tutorial.6592/#content
B4i iLocation example: https://www.b4x.com/android/forum/threads/location-gps.46148/#content