How do I rematch the Gen 3 gyms in Pokemon Emerald?

Question

I've completed my Pokemon Emerald, and my team is level 100 and I wanna know how I rematch the gym leaders.

Answer 1

First you have to enter the hall of fame.
Then on every 60th wild encounter, every 20th Trainer battle, and every 20th Battle Frontier battle, the game has 31% chance to make a Gym Leader rematch available.
You can check if they are available for a rematch by using the Pokénav, if there is a square pokeball next to their name a rematch is available.

You will have to complete the new Mauville side quest to rematch Wattson.

Source

Comments

Also the gym leader's teams will be around level 50 and get higher as you rematch them more so with a level 100 team it will be easy to beat them.