The final information on the crash almost reads like they hit the very top of the mountain - according to NTSB, they were about 200-250 ft from the summit, so it is possible they did still have visual with the airport until right before the impact. Whether or not GPS could've helped with this, I'm not sure - it might've allowed them to see where they were relative to the mountain better, since it was the middle of the night and so they probably couldn't see the mountain very well..