The GPT-130 displays a number of alarms, beside the ground fault it will also alarm for low current. If you have disconnected the load to trouble shoot a possible ground fault in the heater cable, then you probably have a Low Current Alarm and a very low default ground fault current reading.
You can tell the difference by looking closely at the display, it will tell you what the alarm is for. If you want to clear the low current alarm so you can test the controller then go into the configuration menu and disable the Low Current Alarm.
Remember to re-enable the Low Current Alarm when you are done testing and ready to put the unit back into service.