Incidentally, the licensed guides at Gettysburg are not your run-of-the-mill tour guides. They have to pass a rigorous application and testing process to be licensed, which is a pretty fascinating story by itself. For example, I copied the following from the Gettysburg Foundation website, in regard to the exam:
"Recent tests consist of approximately 220 objective-type questions, true-false, multiple choice, matching, fill-in-the-blank,/ short answer. There may also be photographs of monuments which you will have to identify, photos of officers or other personalities both north and south and a map section where you will be asked to identify place names. You also will find anywhere from four to six essay questions designed not only to test your basic knowledge but your ability to express yourself. This is administered in a testing center with about twenty guides as proctors. It is graded by the park staff with the objective part rated numerically and the essays scored pass-fail."
And also this:
"It is important to note that successful guides are those with a grasp of political, social, economic perspectives related to this era. Knowledge of military aspects alone normally is not sufficient. You’ll find that those with the most detailed down-to-the-company knowledge level of the battle usually have the hardest time making it through the licensing process because they know too much of a specialized aspect of the battle (i.e. tactics) and too little of the broad scope of the battle; the human element and the meaning of it all."
So whether you take a bus tour or a private vehicle tour, as long as you have a licensed guide with you, you can guarantee that the guide will have an exceptional level of knowledge.