go to the Pokemon league north of sunyshore
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The 8th and last gym is in Sunyshore City.
It's in the gym when you're done defeating the leader he will give you the TM (8th badge)
where can i find the second gym in Pokemon platinum
You can use the GTS in Pokemon Platinum after you get the first gym badge from the Oreburgh City gym.
West of the hotel where the guy used to say there was a power outage.Go down the stairs and after a few trainers,you will be at Sunyshore City where Volkner's Gym is.