The GPS is a true life saver in a loss of indicated airspeed. When AF lost their airspeeds they only had to look at ground speed that existed at the time and then keep it there. It would have been around 440 since they had a bit of a headwind at the time I seem to remember but it doesn't matter. In still air it will be 480, and in a 200kt headwind it will be 280 but all you do is keep it there.
Your forecast wind on the flight plan is dead accurate these days and so all you need to do all the way to Paris is to subtract or add the headwind to find out what GPS groundspeed should be and fly that speed. You won' t be more than five its off your target. A lot better than what happened - being 350 kts too slow!
John