If Gettysburg was lost, their was nothing to keep Lee out of DC. The Notrth would be forced to sue for pease otherwise DC would go up in flames again.
Lee was not Sherman, there would have been a brief occupation but no burning, like Atlanta. Urban warfare was rare back then. Looking back nothing was going to make the Goon come to the table and the South did not have the strength to hold any Northern territories. Mr Lincoln’s War would have no negotiated ending. The butchery would continue, actually it picked up speed after G-burg.