The Battle of Gettysburg was significant because it defeated a Confederate invasion of the North that would have given the Confederates a strong chance of winning the war.
The Confederate invasion of the North was not really meant to conquer the North. There are those who believe that it might have been able to capture Washington D.C., but this was not really the major aim of the invasion. Instead, the South hoped to turn Northern attitudes against the war and to gain support from France and Great Britain. By invading the North, the South hoped to make more Northerners feel that the war could not be won easily enough to make it worthwhile. The South also hoped to convince the British and the French that they had a chance to win. If the European countries came to believe this, they might have recognized the independence of the Confederacy. This would have put much more pressure on the North to end the war.
However, the South was not successful in their invasion. The Confederate army lost at Gettysburg and had to retreat. This destroyed what might have been the South’s best hope for winning the war.