Gettysburg was a city in Pennsylvania, in the United States of America. From July 1st through July 3rd, 1863, a major battle in the American Civil War was fought near Gettysburg and resulted in more casualties than any other battle of the war. The Third Doctor and Charlie Fisher mapped the Confederate lines and delivered the information to the Union Army, which helped them win the battle. (COMIC: Backtime)
Shortly after the battle, President Abraham Lincoln delivered his Gettysburg Address. The First Doctor, Ian Chesterton, Barbara Wright and Vicki Pallister watched the speech on the Time-Space Visualiser. (TV: The Chase)
Real world location
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map at right is probably a good indicator of the DWU location of Gettysburg.