The GFEP indicator will come on when a ground fault occurs in one of the heater circuits. The insulating resistance of the heater cable is breaking down or the cable has been damaged allowing current to pass to ground thru an incorrect pathway. Left un-repaired the cable could cause a fire if it continues to be powered.
When a ground fault is detected the controller will not allow power to be applied to the heater circuits until the problem is corrected. You can isolate the offending heater by removing all the heaters and reinstalling them one at a time to discover the heater with the fault. An alternative to this method is to use a Meg-ohm tester and test each heater leg per the manufactures specifications.
You will need to contact the heater manufacturer for instructions on how to locate the problem and fix it. Most heaters can be repaired with repair kits or splice kits but the manufacturer will be able to provide the details.