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There is a map of Mt. Coronet in the Sinnoh region.
The 8th gym is in Sunyshore, but you will be unable to go there until after you beat Cyrus on Mt. Coronet.
Mt coronet is only in diamond or pearl or platinum not in firered.
You have to go to the top of mt conoret, where you'll have a battle with your rival against commander Mars and Jupiter. After that you'll go to the distortion world where you have to beat Cyrus and fight Gratina. Once all that is completed, team Galactic will be gone from mt Coronet. To find Cyrus and the top of mt Coronet: http://www.mahalo.com/Pokemon-platinum-walkthrough-part-6/
you can find Cyrus at the entrance to Mt.Cornet near oreburg city and at the top of Mt.Cornet wich is called Spear Pillar.