After you beat you game, go back to sendoff spring. Cynthia will have left so you can enter the cave she was blocking. Enter Turnback cave just like in Diamond / Pearl. At the end you find an item and the entrance to the Distortion world. Go through it to find the Griseous orb. It's like the Adamant orb for Giratina. If you give it to Giratina, it can stay in its origin form outside the Distortion World. If you didn't catch Giratina in the Distortion World, you can catch it in Turnback cave.