GPS works on really weak signals from satellites. If you are able transmit a stronger signal at the same freqency, then you might just be able to seduce the receiver into using that signal to calculate an erroneous position. Bear in mind however, that GPS equipment does not simply receive one signal, but multiple signals from all the satellites that currently above the earth horizon. The algorithm that sorts out these signals is capable of voting out the data from any satellite that does not conform to it's known disposition of satellites. There is also the fundamental problem of the aircraft being closer to the satellite than any ground-based jammer, so the relative power comes into play.
The upshot of all this is....... Whilst it is quite easy to jam a GPS receiver and even make it useless, causing it to indicate a particular spoofed position is almost impossible.
What is more worrying is the possibility of interference to GBAS systems. These have a ground-based transmitter that augments GPS signals to permit RNAV precision runway approaches. As the replacement for ILS, GBAS permits RNP APCH and RNP AR APCH with a performance as good as ILS Cat 1. In fact, complex approaches can be defined to guide the aircraft down valleys and avoid noise hotspots in a manner previously impossible with ILS. Clearly, interference with that system is highly undesirable so it employs suitably encrypted data.