There are several ways to visit Gettysburg, and the self-guided tour is just one of them. They do offer guided tours with a licensed battlefield guide, for an extra charge. There are also a number of commercial tours available in the area that claim to have a historian as tour guides. Check the link, which goes to the NPS site, and scroll down for further links.
When we were in Gettysburg last summer, we used a number of different materials: a book, the NPS paper tour guide, read almost every sign we saw. It was very helpful (to me) to watch the 90s movie GETTYSBURG before we left, to watch the incredible movie that is held in the Visitor Center, and to see the Cyclorama presentation. Do all that first before embarking on any type of tour, be it self-guided or historian-guided.