After the Civil War, what happened to the United States's Gross National Product (GNP) ?

During the war, the economy is put on a war-time footing with production aimed at the war effort. When the war end, there is a massive dislocation as the economy has to shift to peacetime. So even if there was major growth during the war, and even if the transition is fairly short, just after the war the economy sees a drop in GDP on both sides.