The FRA has no specific rules on the use of GPS - I just checked, so it's down to the rules of each race. For most fell races there should be no need to use GPS, and it's probably a disadvantage anyway. Your GPS won't tell you the best route / line: recceing and / or map reading will. If you're following your GPS straight to a checkpoint, beware cliffs, rivers etc! There's a lot more to fell navigation that working out the direct route to the next checkpoint![]()
If you really want to use a GPS on the fells I'd say learn to read a map and use a compass first![]()
For mountain marathons etc, Toreador has given you the lowdown on legality.
Nice list Toreador. There are also some single day fell navigation events that are worth doing e.g. Kinder Trial, New Chew, Lakeland Mountain Trial, Warrior O-Trial (on the orienteering calendar) etc.