After you beat the canalve city boss you go the library and go to the top floor there u will find the professor and friends he will talk to you and tell you to do stuff and eventually after you have gone to all the lakes and done the stuff you need to there you go to the galatic HQ in Vielstone city you beat the boss and then follow him to the top of MT. Cornet and battle Dialga (PS you can catch him with the masterball) go to Valor lakefront and now the dude that was blocking the path out of the right of town is gone! follow that path until you come to a city this is Sunyshore City where the 8th Gym is.
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in the grass in the last city the 8th gym leader
In the lower right side of Sunnyshore city
The first gym in Pokemon diamond is in Oreburgh City
The 8th gym is in Sunyshore, but you will be unable to go there until after you beat Cyrus on Mt. Coronet.
After you defeat the 8th Gym leader you can get HM08 from the Jhoto person standing on the Sunyshore beach.