Just keep going through Sinnoh and don't worry about the gyms. You will totally forget about that because much better things are coming your way... simply keep going through the towns and cities. :)
~Lucario
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In Diamond, Badge 5 can be gotten by defeating Fantina in her Gym in Hearthome City.
the seventh gym badge snowpoint gym
beat the league
in the forest by the earth badge
when you get your seventh badge and you will head for mount cornet