I beg to differ, the gas cap on your vehicle is vented. The next time you put gas in look at the underside and you will see. If your gas tank was not vented, it would create a vacuum as fuel was drawn from the tank into the engine... creating a suction that would make it very difficult to remove the cap, and if you ever did get the cap off, air would rush into the tank! Not good!!!
Modern truck and car gas caps are vented with a damper, it lets air in but not out.