University of Texas demonstrates GPS signal spoofing quite dramatically, by sending a private yacht off course and thus “hijacking” it.
Another source with an ad wall and less technical detail but with the following key quote:

These consumer spoofing devices, the sale of which has been banned in the U.S., can still be legally purchased in the UK, and are available for as cheap as $78 (£50).
And, of course, North Korea has already experimented with the technology, reportedly blocking GPS signal in South Korea on several occasions. One such attack launched in 2012 affected 1,016 aircraft and 254 ships.