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.
I would feel better with an ILS beam, to be honest.