This is what I wonder about. Do GPS receivers, aviation class ones, just accept signals coming from whatever location (maybe not from the sky), at whatever strength and reporting whatever location? Also I imagine inertial navigation still works. Using a last "known good" position, can that be used to do a sanity check and too weed out bad actors?

It's like the internet all over again, where adversarial players were just not considered.

It likely is the case that most GPS receives, even aviation class ones just accept signals.

Until a couple years ago this was a theoretical issue that few real world people ever dealt with. Military class GPS likely has some form of protection (but what is classified) since military use of jammers have been known for a few decades - but they were very limited use and so non-militaries just ignored them.

Today things are different. Russia's military is jamming GPS on a large scale and that is affecting areas outside the war zone. GPS jammers are also cheap enough that the FCC and similar organizations (most non-poor countries have one) are unable to keep up with all the illegal uses (most countries have laws predating GPS that make these jammers illegal)