I understand the wrong turn and all, but how can he "forget" that he had a gun in the console? Anyone who "forgets" where they put their gun clearly does not have the mental capacity to own one.

If he had declared the gun when he approached the checkpoint, he would have simply been turned away. This isn't about a wrong turn or a faulty GPS, it's about his losing track of his firearm and not realizing he had it in the car, which is unacceptable no matter where you are.