Gary Bryman was a victim of a hit-and-run.
When Gary was eight years old, someone driving a black Toyota Camry ran him over, killing him. The driver fled the scene and was never caught since no one saw the driver's face or remembered the license plates.
No one knew Jill was behind the hit-and-run until Sam and Dean Winchester searched her room and found Gary's name written on the back of the bathroom mirror. Charlie recognized the car involved in the accident as the same one Jill drove.