The simple GMC Gas Cap is a very misunderstood component on your vehicle. Most GMC truck owners know the gas cap is the cover that prevents dust and debris from entering the channel that leads to the fuel supply on their vehicle, but that is not its only purpose.
The gas cap mounted on the exterior of your GMC vehicle can have a locking mechanism contained in it or can be of a simple twist off variety. Immaterial of the style, it will also have to have a polymer gasket on it so a seal can be made at the entrance to your fuel tank. This will prevent the accidental leaking of gasoline or diesel fuel from exiting the tank when the truck is in motion and an obstacle is encounter by the driver.
In the center of the gas cap is a small hole. This is the vent system for the fuel tank. This small vent hole is generally obstructed from view by a cover so fuel will not exit the hole, only air or fumes from the gasoline. This vent in the GMC gas cap will prevent the buildup of pressure in the fuel tank. If the vent is obstructed for any reason and pressure is permitted to build up in the fuel tank, the fuel pump will be the component that receives this burden. The most common result is the rupturing of the seals in the fuel pump causing the component to fail.
To prevent the chances of an obstruction interfering with the vent in the gas cap, it should be inspected regularly on your GMC truck. It is generally rust that causes this obstruction that is formed from the moisture in the fuel tank. If this rust is noticed, the cap should be replaced.