It is the GMC Thermostat that regulates the flow of coolant out of your engine so the correct operating temperature can be maintained when it is running. The opening and closing of the thermostat is regulated by the bimetal spring on its backside facing the hot engine coolant in your GMC truck.
All GMC thermostats are located at the top of the engine block with a housing unit covering it. This housing unit has a short tube on it that makes it possible for the upper radiator hose to be attached to it. If you are unsure of the location of this temperature regulating device, just follow the upper radiator hose from the radiator to the GMC engine block. The housing unit is generally held in place by two bolts. Once the housing unit is removed the thermostat will be visible.
The failure of a thermostat is only noticed when the engine is either overheating or never obtains the correct operating temperature. If the engine is overheating then the valve in this temperature control device is jammed closed and is preventing the flow of coolant from the engine. If this occurs when you are on the road, the upper radiator hose can be removed and a screwdriver can be used to open the valve. This is a temporary fix until you get to a place where a proper repair can be done.
The manually opening of the valve on a thermostat will make it be the same as if the valve is stuck open and not permitting the coolant to reach the correct operating temperature. In this condition not only will the fuel mileage be negatively affected on your GMC truck, but the heater in the passenger compartment will also not be able to generate enough heat for the occupants in the winter time.