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.

We also have visual navigation systems that can use satellite imagery to produce guidance packages. They will become increasingly less useful the longer the imagery goes without updates after a Kessler syndrome type event, though. Another strategy, which I believe the Ukrainians have been experimenting with, is scattering small radio beacons throughout the battlespace to provide a network of navigation aids.

In a true Kessler scenario where LEO is denied for an extended period of time, I expect a variation of that strategy is how we would replace GPS for most civilian use cases. Likely by using cell towers to triangulate, possibly enhanced with additional transmitters to improve accuracy and coverage. Pure cell tower triangulation would have much worse coverage than GPS, obviously especially in rural areas where cell service is often already poor, but also anywhere that getting a signal from multiple towers isn't easy or possible. Cell towers are never distant enough to use the relativity-based timing tricks GPS relies on, so multiple signals would be required to triangulate. We'd likely end up with substantially less precise positioning, maybe something more on the order of the 10-20m of error we used to see with GPS before the US stopped limiting civilian access to the deaccuratized version. Cell tower triangulation also obviously doesn't work for aviation or oceanic shipping, where trident-like inertial+stellar nav would probably be the only workable option.

Actually it hasn't proliferated into civilian use because the military industrial complex has captured pretty much anything with real accuracy.

I had an old colleague who in a previous life worked on an incredibly accurate positioning system that was effectively a brief case that knew where it was at all times. The story he told was the DoD made them an offer they couldn't refuse, and everything became secret. I'm pretty sure the company in question is still in business.

I'm not trying to say it's not legal but it might not exactly be legal?