You cannot fight the Gym Leaders again in Pokémon Black after defeating them and being awarded their Gym Badges.
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i think u have to fight them and
You can't
Use Thief or Covet on the Machamp the Black Belt Nob on Route 115. At the fifth battle and the battles after that, he will have a Machamp. To get him to fight you again, you may try to go into the cave and then back out again a few times. It may help, not guaranteed to work. You can get several of this item, all you have to do is get him to fight you again after the fifth battle and keep stealing the item. Make sure that the Pokemon who uses Thief or Covet on another Pokemon does NOT hold an item before trying to steal one.
you can go after the legendary pokemon, or beat him again.
After you win three sets of 7 battles, you'll face the leaders of the facilities.