"Take that hill if practicable." The battle might have gone much differently if Lee had given more forceful orders. Lee was a great general fighting a war against long odds. Gambling was necessary to achieve victory but he should have listened to Longstreet at Gettysburg. The Confederacy simply didn't have the manpower to continually fight long battles of attrition. By the third day Union soldiers were crying, "Fredericksburg!" because their positions were reversed: it was the Confederates marching uphill against an intrenched position.
A Southern victory at Gettysburg is one of the most popular topics for alternative history speculation. Newt Gingrich and William R. Forstchen turned it into a trilogy of novels. I think they spin the most likely outcome of a Southern victory: the Confederacy still loses the war and sooner besides. It was the fall of Vicksburg in the West that sealed the Confederacy's fate.