Although there are many interesting things one could do with a GPS signal with encrypted side channel, persistently skewing measurement in one area through spoofing would be hard.

GPS and similar geolocation systems work through precise time-of-flight measurements using broadcast time signals from atomic clocks in satellites. Receivers get both satellite location information and "what time is it" information from each satellite. To spoof this in a way that would be seamless for a receiver moving into the distorted location, you'd need to produce a more powerful spoofed signal with distorted time or trajectory information that would match the currently available constellation of satellites well enough that the GPS receiver wouldn't loose its fix. You'd also want to keep the overall time signal close enough that local time wouldn't appear to jump too much.

If you don't care about the problem of what happens when the receiver moves into or out of the distorted region or matching the current time, you can broadcast whatever kind of spoofed signal you want, but this may not be very convincing for an adversary who was paying attention. It would be a lot easier just to locally jam the signal.