I think the general assumption is GPS is denied in most combat zones, it's constantly being jammed in and around Ukraine.

For most things the fallback is going to be everything we used to use. Maps, stellar navigation, inertial navigation that sort of thing.

Guided weapons wise, terrain mapping is pretty common for cruise missiles. Storm Shadow, for example uses GPS, terrain and inertial guidance systems. Trident uses inertia and stellar navigation.

As for why it hasn't proliferated into civilian use? Mostly it has its just the old things. You can perfectly well drive your car with a road map and sign posts. Pilots still train to use dead reckoning and beacons. Etc.