As to the action, this is a link to what has been reported as to North Korea jamming South Korean GPS signals.
On average, there are 28,500 American soldiers stationed in South Korea, properly known as the Republic of Korea.
Those soldiers use Global Positioning System devices rather a lot in order to support their combat potential, as do the South Koreans in the same types of applications as we do, aircraft, maritime navigation, even automobile navigation.
Attacking such systems is annoying, although not generally speaking life-threatening, if it must be taken at an absolute minimum as a signal of the sort of thing the North would definitely be trying to do in wartime conditions.
We could expect cyber attacks here too, and of course such North Korean activities are why many don't like their satellite programs, as they can be assumed to have a military and hostile intent.
As to how much too worry, some have worried that American forces are too dependent on GPS, although that could be exaggerated.
For example, although J-DAMs wouldn't work well with GPS jamming, W-88s launched from Trident submarines most definitely would eliminate any enemy targets that were to be engaged with a J-DAM, as would B-61 tactical nuclear weapons dropped from F-16s, or even just area raids with B-1Bs and B-52 H's, if the loss of North Korean non-combatant life would be higher in such a scenario.
North Korea is many things, risk-taking among them, but as to a total failure of rationality, that cannot be said to be generally true. They want to reunify the Korean Peninsula on the terms of their social system, and are willing to use force to do that, it's just that they can't do that, since we would in the end use Trident submarines to efface them from the Earth.