That's pretty incredible for those small planes that I doubt have all the sensors jets do

You'd be surprised. Typically, you'll find that jetliners are older and therefore have an older tech stack, but also that they're redundant. Multiple pitot tubes, for example. On newer GA planes, there may be less redundant sensors, but the technology running them can be more modern and more powerful. The tools on the jet get the job done with two pilots and are there to make money, while the GA airplane is going to be piloted single pilot, and more likely for pleasure.

GPS gets you 99.9% of the way there. Runways are 100+ft wide. 10ft precision is fine.

If the runway does happen to have ILS, there's another authoritative source of information too.

Moreover, high end dual frequency GPS receivers like you'd surely put in a plane will typically have sub-meter precision.

Depending on the environment? GPS is still susceptible to signal reflection and LoS blocking.

Sure, depending on the environment. But when an aircraft is in one of those environments where reflections and line of sight to satellites are an issue, GPS accuracy is the least of your concerns.

This is in regards to an emergency response system, so an attacker would have to wait for an emergency to attack and well, you may as well just see how it pans out naturally rather than risk jail time to try and coerce an accident that might already be happening.

It’s not (just) about attack though. If you have your emergency in a place where the surrounds cause GPS to be inaccurate you cannot rely on GPS. Ergo since you cannot plan where to have an emergency relying on GPS is not sufficient.

I don't believe LoS blocking is really an issue as you approach an airport runway from above...

If you can make sure you only have emergencies when near an airport then you've already solved a lot of the hard problems.