If in fact you did have a heavy load on your GMC Sierra during the smog test it may have caused it to produce high emissions. We'd first recommend getting the smog inspection with the bed empty. If your GMC Sierra still fails the smog test you'll need to have a hands on diagnosis conducted in order to find out what's causing the smog check failure.
Smog check failures are often due to defective emission sensors such as the Oxygen Sensor(s), Mass Air Flow sensor (MAF), and ECT (Engine Coolant Temperature) sensor.
You'll need to have smog check repair station will find out if your GMC Sierra is in proper fuel control. The diagnosis for this fault should begin with the inspection of the fuel injection "feedback" system. This includes ensuring all the emission feedback sensors (such as the the O2 sensor, MAF sensor, AIS sensor, and ECT) are working properly, your vehicle's emissions computer is receiving the signals from these components, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers.
