What effect would an air-to-air GPS jamming system have?
Jamming GPS means that the GPS receiver is unable to lock-on to the signal from any satellite.

Presumably, pilots of small aircraft would have to revert to more traditional forms of navigation - assuming they have a GPS based navigational instrument or two.

Pilots of airliners would presumably see alerts and (as with most smaller aircraft?) their flight displays would include non GPS sources such as gyrocompasses or magnetic compasses.

I imagine redundant systems would include other sources of position information such as reception of radio-based omnidirectional navigation beacons, inertial navigation systems and looking out the window.