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 )
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No unfortunately :(
go to serebii.com it tells how
I heard the walkthroughs from youtube but you can't
Unfortunately, you cannot. Once you defeat them, you cannot have a rematch. However, if you want a better challenge, you could rematch the Elite Four. They will definitely level up your Pokemon.
you can't but after you go through Stark Mountain and reach the Survival area at the club you can rematch theminside