You go to the 8th Gym, which is in Sunyshore City. Not exactly you can't go there until after you've beaten or captured the legendary of the game depending Dialga>Diamond or Palkia>Pearl or Giratina> Platinum, that are on top of Mt. Coronet. Hope i helped!
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In Pokemon Diamond\Pearl, you go to the Veilstone City Gym. In Pokemon Platinum, you go to the Hearthome City Gym.
beat the 7th gym leader and then go to mt cornet he wiill not be there
go into oreburgh mine and talk to gym leader
Go to herthome and beat the gym master -Game master
you go to sunnyshore city but don't go to the gym go to the lighthouse and you will find the gym leader talk to him and then he will go back to the gym so you can fight him