It's not griantina, but Giratina. And it's turnback cave. Just look at your map. In Route 214, you can find a secret entrance at the east after beating the E4. Just follow the path until you get to a very confusing cave. I don't know how do you know path, I just walked until finding Giratina. Then catch it and that's it.
EDIT:TwilightBlade, you beat me for seconds!