I’m not sure if they still use it, but there was an Inertial Reference System on board - you set your location at the start of the flight, and then it can (roughly) give you a position using dead reckoning. A large disagreement between this position and the GPS position would result in an alert.
Definitely still a thing and I think that’s the goto when GPS fails. Of course there’s drift that accumulates. I think the primary threat to commercial aviation without GPS is guided landing systems where you need the location precision.
