GPS is extremely low power and relatively speaking easy to jam. The received signal strength is far below the noise floor to begin with and it's basically RF black magic that it works to begin with. Those cheap GPS jammers that only radiate white noise at like a watt cause huge problems in the surrounding area.
There was even one case that I know of that would pertain to this subject in particular. A truck driver wanted to prevent his company from snooping on his whereabouts while driving a truck with a tracking system on it so he bought and used one of those cheap chinese jammers. He delivered stuff to an airport and just sitting in the parking lot was enough to keep aircraft from maintaining a GPS fix. There was also the case at Newark airport where just a truck driving by regularly was enough to cause problems.
And really those GPS jammers are the bruteforce and pretty ineffective approach. White noise is essentially what GPS is designed to work in, it just raises the noise floor 20 - 30 dB. GPS uses a gold code to transfer information and it's essentially looking for statistical deviations to be able to get a lock on such a weak signal. If a jammer was designed to be biased and transmit with the intention of preventing a receiver from being able to lock onto a satellite instead of just overpowering the signal entirely it could jam more with less power.
https://en.wikipedia.org/wiki/GPS_signals#Coarse/acquisition...
https://gizmodo.com/jamming-gps-signals-is-illegal-dangerous...
https://www.cnet.com/news/truck-driver-has-gps-jammer-accide...
Example: http://jammergroup.quality.chinacsw.com/pz66df87e-handheld-6...
And by the way, Boeing had jamming resistant GPS back in 1998.
https://boeing.mediaroom.com/1998-04-23-Boeing-Anti-Jam-GPS-...
To me any concern about GPS jamming is plain bullshit added to the soup to justify further restrictions on drone flying. Drones with cameras are powerful weapons these days.