Besides the battle losses, the period around the battle of Gettysburg had two important strategic effects. 1) It established the winner, George G. Meade, as the General of the army of the Potomac. 2) More to the point, it established U.S. Grant, who captured Vicksburg at about the same time as Meade's boss.
The Army of the Potomac began the 1864 campaign with the battle of the Wilderness, suffering losses at worse than the two- to one ratio by which the North outnumbered the South. This qualifies as a major Confederate victory, and under different circumstances, might have helped them win the war.
Meade actually wanted to retreat, like all his predecessors. But when the Union army marched east to reach the main north-south road, Grant stood in the intersection, facing west, with his left hand pointing SOUTH. This forced a followup battle at Spotsylvania, where the casualty ratio was about one to one.
This gave Grant the idea of forcing battles at a one to one (or slightly worse, for the Union) casualty ratio. When questioned about his bloody tactics, his reply was "I plan to fight it out along this line if it takes all summer." The South could not survive such a war of attrition. Can One Use Attrition Tactics When There are No Other Clear Means to Win a War.?
The battle that might have decided the war in favor of the South was the battle of Chancellorsville (just before Gettysburg). There, Confederate general "Stonewall" Jackson conceived a bold plan to cut off and capture the entire Union army after a successful initial flank assault. In reconnoitering the ground (at night) for this plan, he was shot by his own men, who mistook his blue overcoat for a northern uniform. This cost the South both a lost opportunity, and the services of Jackson himself.