it could be a sign the car has been rear-ended, so check carefully for evidence of this: poor panel alignment, mismatching panel colours, overspray, and look under the bumper/underneath.
the electrical contact/switch for the rear hatch is built into the actual lock mechanism on the hatch. if the electrics are otherwise ok, then the switch mustn't be activating properly, possibly due to poor alignment. again, may be a result of an accident, but another possibility could relate to squeaks and rattles in the GT. this can be due to poor hatch alignment too, and an attempt may have been made to fix the squeaks, resulting in the 'lamp' problem.
check the reverse parking sensors carefully. as above, may be a result of an accident, damaging one/them. but they can also become unseated from being pushed, or even high pressure car wash jets of water. they are clipped into position, under spring tension. a clip may have broken, and a quick fix with silicone or similar may have been performed, upsetting the sensor. also check the wiring feeding to each sensor: quickly inspect if it's in place and not disconnected (probably too hard to see).
another cause can be a big rear number plate that overhangs across a sensor. check the 2 'middle' sensors near the number plate.
so long as the car hasn't been in a major accident, it's probably allright, and easy enough to have fixed.
there are other threads in the GTA/GT subforum about what to look out for on GT's. in general, a very solid car.