You will need to defeat Volkner, the Sunyshore City Gym Leader.
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8th gym badge the water badge
by beating the gym leader
Hearthome's badge
you have to use the HM surf and surf behind the gym and go in the cave. go to the other side of the dragon temple and go in. awnser the old man's questions and the gym leader will come in and give you the badge. This is to get the 8th Johto gym badge. To get the Kanto one search: how do you get to the 8th Kanto gym badge
You can use the GTS in Pokemon Platinum after you get the first gym badge from the Oreburgh City gym.