You can OC in Gettysburg, I've done it several times, no one cares. The police are aware of your rights and generally won't bother you. If you want to OC while driving, though, you need a PA LTCF or one from a state that PA honors. As for the GNMP, you're safe to open carry anywhere on the grounds, and in structures out on the battlefield, but I'd be careful about the visitor center itself. Techincally speaking, the law says that on National Park property, the gun laws from the surrounding state apply, so you should be ok in the visitor center, but I haven't been there in a while and don't know if they have a no guns sign.
Generally, though, you can OC in Gettysburg no problem. Maybe I'll run into you sometime!
Park grounds are okay, as is all of "civilian" Gettysburg. The Visitor's Center, however, is off-limits for carry. The 18 USC § 930(a) prohibition applies since the building contains federal employees. It is posted. Rest rooms, however, are okay.