Gettysburg was a turning point in the Civil War because, after the amazing number of deaths, it made General Lee decide to never invade the north again, and decide to resign.
The reason Gettysburg was so important was because it respresented General Lee's last attempt to invade the North. He knew that if he could win big battles in the north, that the Confederacy would get respect and destroy the Union's spirit to fight.
What he did not know was that the war effort was not going very well in the north and that the Union victory at Gettysburg would give Lincoln everything he and his generals needed to keep fighting the Confederacy.
After three days of battle that Lee's army had success, Meade's army was still holding its ground, the not smart assault by 12,500 Confederate troops on Cemetery Ridge was the straw that broke the rebels back. The Union line held while pushing the Confederates back, and the Confederates suffered severe loses.