The battle of Gettysburg was the the turning point in the Civil War because before this battle the South had been slamming on the North relentlessly. Then, this battle was a victory to the North and the South never won a battle since.
The two battles that coincidentally happened at the same moment, although far apart - Gettysburg in the East, and Vicksburg (probably the key event) in the West.
Because it ended the war in the West, enabling Grant to come to the aid of the Army of the Cumberland.It also liberated the Mississippi, cutting-off all Confederate units to the West of the river.