I think it's a kid thing, not a gifted thing. Some kids have accidents and others don't. My ds had his last accident at 4, when he wet the computer chair. I cleaned it up and told him he couldn't use the computer again until the chair dried, and that would take a week or so. Once he had his computer privledges back, he never had another accident.
If your ds has accidents in school, make sure he has a change of clothes at school or in his backpack. Teachers don't like to deal with it, but I guarantee your child is not the only one in his class whose had an accident this year. I work at a school, and I know of at least 2 3rd graders who had accidents this year so far.
