What are the ICAO GNSS waypoint naming conventions?
There are no "GNSS waypoints". GNSS is a common position source for RNAV, though not the only one.

All RNAV waypoints, like conventional (e.g. VOR/DME) fixes, have an identifier composed of five alphanumeric characters that is unique within a country. Some countries don't use numbers (e.g. the US); others do.