The GMC Catalytic Converter is nothing more than a large catalyst so as the exhaust gases flow through it, the dangerous oxides can be converted to a less harmful gas. The catalyst portion of the catalytic converter on your GMC truck are the numerous platinum pellets on its interior that the exhaust gases have to flow around to exit this exhaust component.
The reason the catalytic converter on your GMC will be in need of replacement is because the platinum pellets have been saturated with the elements from the harmful exhaust gases and can no longer work as a catalyst in converting the gases to a less harmful form. This situation will become known to the owner of the vehicle when they fail an emission test.
The replacement catalytic converter can be a new component or one that is refurbished. The refurbished components have had the old and saturated platinum pellets removed and replaced with new ones. The old housing of the converter has to be removed from the vehicle to perform this procedure. This is required since the hole to remove and replace the platinum pellets is on the bottom. It is possible to remove the old pellets with the converter in place but placing the new ones inside of the housing unit is not possible because of the position of the hole.
The GMC catalytic converters have also been known to become clogged when bad fuel is used in the vehicle. This condition prevents the escape of the exhaust gases and will reduce the power your GMC truck can produce. If this situation is not rectified the engine might no longer be able to run.
A catalytic converter can be replaced by the owner of the truck, but they will have to have a torch and know how to weld so they can attach the new one in place in the exhaust system.