Basically, you have to get their phone numbers. Then, you have to call them at the appropriate time, and they will stay at the dojo for a rematch indefinitely until you meet them there. If you call all of them at the appropriate times, you can get all gym leaders in the dojo at one time.
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Right next to the Saffron city gym. You can use it to battle gym leaders you have already defeated if you have their phone number and phone them when they are not busy. You may not be able to get all of the phone numbers, before you battle Red, and they might all have 6 Pokemon apiece in the fighting dojo battles.
You can rematch all 16 gym leaders at the Fighting Dojo near the Saffron Gym, you may have a rematch against the gym leaders only when you got their Poke-gear No., while they are away from the gym at a specific time each week also the rematch is in a specific time (ex. Whitney is missing from the gym any day on 12:00-16:00, go to the Goldenrod Dept. Store 6th floor and get her number, her rematch call is on Saturday Afternoon )
Defeat all of the gym leaders in Pearl. ~KKMG1
defeat all the other gym leaders
you must beat all other gym leaders