First, GPS signals are relatively weak. Second, they come from 'up' - if you really want to avoid terrestrial jamming, then a bit of shielding that only exposes your receiver to the sky will help a lot.
The solution for creating interference is relaying legitimate signals from space, if you can't crack the encryption. By messing with timing carefully, you can severely degrade the position accuracy or cause it to drift to where you want it.
I find it interesting that GPS, Galileo and BeiDou share 2/3 of their base frequencies, but GLONASS doesn't - its overlap is additional frequencies. I'm not a comms guy, but I do wonder if that means Russia can interfere with GPS, Galileo and BeiDou simultaneously without affecting their own gear significantly.
It means less and less when it comes to military use though, since the military expects jamming and spoofing and has multiple methods of position fixing of various degrees of accuracy.