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.
Well There is no way to rematch the gym leaders unless if you hack it!
You can't rematch them unless you start again
After you defeat all 16 gym leaders, you can call them to arrange a rematch through pokegear in fighting dojo in saffron city(kanto). See related link below to get gym leaders phone numbers and when to call them.
You cannot.
Hackless, NO!
You cannot.
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 )
america
You'd have to use cheats.
I'm sorry, my last answer was wrong, you can rematch them, but you first have to get their number, and some of them will only rematch you on certain days.
No, this feature is only available in Pokemon Emerald.