I believe that the Gettysburg Address was a turning point in the Civil War because the Confederacy lost the Battle of Gettysburg from a military standpoint. Many reasons were suggested, but one of the main reasons was mental. It was General Lee's only hope to win a big victory in Northern territory, but he lost it, and therefore the morale went down. Lee had to go from offensive to defensive.
The Battle of Gettysburg was CSA's General Robert E. Lee’s second invasion of the North. Lee's army was defeated and he suffered more casualties than the south could, that is Yankee homeland. About 25,000 of his 75,000 man army were casualties. After Gettysburg, General Lee ever had enough men to attack the North directly again effectively creating a military turning point of the war.
Their loss at Gettysburg had a huge impact on the Confederacy economically. Once news of their defeat reached England, they pretty much lost all hope of having Europe recognize them as an independent country. Also the Confederacy lost the Battle of Vicksburg, and after that, the price of Confederate war bonds dropped significantly, because it was the first time that people recognized that the Confederacy's efforts were failing.
It was a significant battle in terms of the Union, as well. It served as a boost of confidence for them, proving that they could win after previous losses. It also proved, like it did to the rest of the world, that the Confederacy couldn't survive as it was. They couldn't keep a country on farming, which had been severely damaged with the war in the first place.