Hello,
Have two PBP (2007 and 2011) under my belt.
Have never taken a GPS and only taken a route sheet. The signage is so good I didn't even look at the route sheet. If you come off at the incorrect exit to a roundabout, there is even a sign with a 'X' on it saying you are going the wrong way. I my opinion you don't even need a GPS. I would imagine you will get more lost trying to find the start of the ride.
Just enjoy the ride, eat lots of French food, cidre and make friends from all over the World
Joolz