You know what it says on the charts
about "The Prudent Navigator"...so if the same government
that runs the gps systrem, says the gps system may be messing around with you for a couple of days, better to presume your gps IS in fact lying, until and unless confirmed otherwise.
The cell phone
and GPS jammers that you can mail order all have very limited ranges, typically 20' to 100'. And there's a nice stiff penalty ($5000+) for using one. So they're unlikely to be a big domestic problem.
And as the folks at alt2600 pointed out some years ago, the nice thing about a gps jammer is that it sends out a strong active gps signal. Which will
promptly be targeted by a weapons system if it pisses the wrong people off.