If you have beaten the 7 gyms, your next step is to rescue the spirits from the lakes.
Next, head to Mt. coronet - Finish the place and defeat The Bosses and grunts.
Then, you head for distortion world - a mase like place where Giratina lives in. Finish the place and battle giratina ( you should capture it ).
You will end up in sendoff spring. After this, the man blocking the way to Sunyshore City will be gone.
Now you have unlocked Sunyshore, where the last gym is. Beat him and you get accses to the Pokemon League.
Hope this Helps....... :)