IF the pit trips the gfic the instant it is plugged in, unplugit from gfic then unplug all the circuits: Blower fan, Auger motor and Igniter. This leaves the only the controller connected..Plug the unit in and see what happens when the pit is turned on. If the gfic trips, the controller is the cause.... If not, start plugging in one circuit at a time until the gfic trips. The circuit that causes the trip is the culprit....
BEFORE starting the above test - try another device in the gfic to see if it trips. If it does the gfic is the cause..Something else that may trip the gfic is OVERLOAD. The gfic may share a circuit with something else and combined with the pit start up, ~ 400 watts, overloads the circuit. Keep us posted please.