Great discussion.
What worries me about GBAS, is the fact that some teenager hacker seduced or cheated by terrorists or common criminals could somehow send wrong info to the belly GPS antennas. With SBAS, they can't do such thing as the antennas are on top of the fuselage.
As for the GPS signal jamming, I thought that it was difficult, as it is a military designed and the pseudorandom noise is suposed to be difficult to be jammed. I also heard that the soviets (or russians) had jammers for GPS guided missiles. So it is possible, but I don't know if it is easy.
1. Sending false GPS data is possible, but unlikely IMHO. It is true that GPS was designed to be difficult to forge and jam.
Transmitted GPS Signals
GPS explained: Error sources
2. Any signal can be jammed it's a matter of RF transmit power and occupied spectrum. I have seen a modified home cordless phone partially jam a wireless security camera from 50 feet. GPS is vulnerable to jamming because the received signal strength is very low by the time it reaches the receiver. RF is more difficult to jam over a large areas/volumes, easier to jam over smaller areas/volumes.
3. It's not clear that the antennae location makes that much difference. If you were directly underneath the plane it would make some difference. If you were at right angles to the side of the plane, it might may no difference at all.
4. We're headed into a period of heightened solar flares, that is probably my biggest concern with GPS and all satellite and RF based systems.
Solar Storms and You: Human Impacts