So spoofing is complex, but what would the drone do if it didn't have any valid GPS signal?
Without it it would have to rely on dead reckoning calculations, with only air speed, bearing and duration to work with to attempt to find its location. It would be off track and lost very quickly indeed.
In such a situation it could have a fail safe procedure, such as throttling the engine back and performing a controlled descent, or does it just keep zooming along in a lost style, hoping for a GPS lock, until it finally runs out of fuel and drops from the sky?
If it is the former, then spoofing the GPS signal with all its accurate timing would not be required, you just need to drown the signal out with garbage which should be a great deal easier given the weak signal sent from the GPS satellites.