You don't need a GPS on PBP, just follow the signs.
In 2007 I didn't use a GPS, nor did I use the route sheet.
I mainly followed other riders,
and when alone (or at the front) I followed the signs.
However, on the way back to Loudeac, it was VERY dark, and not all junctions were signed (no PBP arrows, and no any other road signs, so the route sheet wouldn't have helped) so I stopped, and waited for others to come and rode with them.
I could have got lost with them, but it was a little more reassuring to have several other riders around me.
Tal.