What are the ICAO GNSS waypoint naming conventions? I'm curious about ICAO naming conventions regarding GNSS and RNAV waypoints. Would I be correct in assuming that all GNSS area waypoints are 5 letters, and RNAV waypoints are 6?

What about waypoints like CB401? Do these represent a reference to a VOR/DME? If so, why are they included in GPS waypoint databases?
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.