DIAGNOSIS The GR2144P draws a lot of power (around 1500-1800 watts). Using a thin, long, or low-quality extension cord creates resistance, which can cause the cord to overheat, the grill to underperform, or the circuit breaker to trip.
FIX You should always plug it directly into a wall outlet. If you must use an extension cord, it must be a heavy-duty cord (14-gauge or thicker, the lower the gauge number the thicker the wire) and as short as possible. Never use a lightweight indoor cord or one that's coiled up.
IF THAT DIDN'T WORK If it's tripping a GFCI outlet in your kitchen even when plugged directly into the wall, the grill itself might have a minor ground fault or moisture inside. Unplug it, let it dry out completely for a day or two in a warm place, then try again. If it still trips, the grill has an internal electrical fault.
HEADS UP This grill, like most high-wattage countertop appliances, is not intended for use with power strips or multi-outlet adapters. Using an inadequate cord is a fire risk. Also, ensure you're not running it on the same circuit as a microwave or toaster oven, as that will almost certainly overload the circuit.