Where is the ASA/CASA written authority to use GPS from the US government ?
Where is the US governments written confirmation that they will compensate GPS users when the GPS signal is 'lost'...
The regs all speak of GNSS (which is a generic term for satellite navigation systems), rather than specifying GPS. GPS is a GNSS system, as are BeiDou, Galileo and GLONASS.
In practice though, most if not all the TSO'd GNSS receivers on the market are GPS receivers, so the fact other systems exist doesn't really matter much!
Interestingly, while AsA and CASA are so confident that GPS will be around for ever that they're happy to put all their eggs into the ADS-B basket, they're not confident enough to get rid of all the ground based navaids. They're going to keep a "Backup Navigation Aid Network" around after most VORs and NDBs are decommissioned, specifically in case of a large scale GPS outage.