At certain times in the day you'll meet a gym leader somewhere outside of battle, talk to them to exchange numbers. Then you can call them to see when they are available. When they are, call them and they will ask you if you want to battle. Accept, and go to the specified area to battle. They have a better team than from your regular battle, Sabrina has a Gallade when you rebattle her. Rebattling leaders also helps fill your pokedex.
Hackless, NO!
Walk in and save the game then go in and in out
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 rematch just any trainer, there are only certain trainers that you can re-battle such as the 4 Elite 4 trainers as well as Alder. Additionally you can rematch the trainers at the Big Stadium and the Small Court in Nimbasa City, you can also rematch Cheren in Victory Road, Bianca in Professor Juniper's lab, Cynthia in her Villa in Undella Town, Morimoto in the Game Freak building in Castelia City and the Royal Unova trainers.
You cant get a rematch in diamond and pearl that I'm aware of. Not sure about ruby/sapphire though..
Well There is no way to rematch the gym leaders unless if you hack it!
You can't rematch them unless you start again
No unfortunately :(
You cannot.
Hackless, NO!
You cannot.
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 )
go to serebii.com it tells how
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.