After Antietam, maybe. The U.K. would likely have stepped in and tried to broker a peace. But by Gettysburg that ship had long since sailed and a rebel victory wouldn't lead to much of anything except another battle in another area.
It always amazes me that people think that the South would have won the war had they won at Gettysburg. By that time, the Northern army was clearly superior in manpower and supply to the Southern army. In fact, supplying his army off the Northerners in PA was one of the motivations behind Lee’s invasion of the North that led to Gettysburg. In the actual event, the Southern armies fought on for nearly two years after Gettysburg. Why would anyone think that an army in better shape than the Confederate army would not have been able to fight on after a loss at Gettysburg?