People were doing the LWW and all sorts of other paths and trails before GPS was around, so realistically, its not an essential item to have. A map and a compass and the ability to use them are however!
Having said that, a GPS can be useful, if for nothing else than to tell you exactly where you are, so you can then switch to map and compass. Even an inexperienced GPS user should be able to get the current position from the device. But practice is the only way to be sure.
Treat it like a pair of boots - you wouldn't consider doing the LWW in a brand new pair of boots without having walked them in first would you?
My GPS walked flawlessly all the way across the C2C recently and I never had to resort to the compass stuffed in the bottom of my pack. But then a compass will never run out of juice.