Right. They have a few hours of holdover before things start to lose sync, and when sync degrades, the towers may stay on the air but handoffs get real rough.

Right. They have a few hours of holdover before things start to lose sync, and when sync degrades, the towers may stay on the air but handoffs get real rough. They may also take themselves off the air after too long in holdover, but that's a software setting. Conceivably this could be overridden by network operators (in close cooperation with their equipment vendor) in a prolonged GPS outage where it was determined that having the towers on-air as islands without handoff would be better than nothing.