By plane
Several airports are in the region, although none of them are particularly closeby. Airports in Harrisburg MDT, Baltimore BWI, Washington Dulles IAD, and Philadelphia PHL are the closest. Flying into Baltimore would probably be your best bet, as the drive to Gettysburg is approximately 60 miles.
By car
Gettysburg can be reached via US Rt 30 from Philadelphia, or via State Rt 97 from the Baltimore area. From the south, US 15 runs through the town for visitors arriving from the Washington DC region.
By train Amtrak has stations in both Harrisburg and Lancaster. Taxi service and car rentals are available to Gettysburg from there.