go to serebii.com it tells how
Chat with our AI personalities
No unfortunately :(
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 )
I heard the walkthroughs from youtube but you can't
defeat all the other gym leaders
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.