It's probably possible to use timestamps, but I suppose you would have to handle ties in more places, with sequence numbers you only break ties once. It appears that the FIX specifications allows up to microsecond precision, but given the volume of messages it's still likely a problem. It's also easier to work with integer sequence numbers than timestamps, but that's also a small consideration.

GNSS is the generic term, GLONASS is the name for the Russian system.

Thanks for the correction. I just know the Russians and Chinese have their own systems.