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.
How to Fix It
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.