I don't think a win at Gettysburg would have made much difference as to England entering the war on the side of the south. The Emancipation Proclamation pretty much assured that England would not assist the south.

Lincoln had written the Proclamation but was waiting for a Union victory to issue it. Even tho Antietam was pretty much a draw it was close enough to a victory that he went ahead and issued it. McClellan had Lee's battle plan courtesy of a Union soldier who found some cigars wrapped in a copy of the plans and still came away without a complete victory.

Common sense and deodorant are a lot alike. People who need it the most don't use it.