Scotty - you pass through a small part of the battlefield, but unfortunately you don't get to do much running there.  Most of the course is through the rural areas around the town of Gettysburg. National Parks can be weird about group events on federal land, though there's a marathon that runs on the C&O Canal and through Antietam and Harpers Ferry, which are all Federal property.  I've run through Gettysburg battlefield a couple of times and really enjoy it, as long as I avoid weekends.  It is supposed to be a fun race, though hilly.  You pick a side, north or south, and the winning side gets a commemorative glass at the awards ceremony, based on the top finishers' times.  There's a half too, but it's in October.  I don't think I'm in good enough shape at the moment to do that, though I'd like to.  .