You cannot rebattle Gym Leaders.
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you cant
You can't. You could use the VS Seeker to challenge Trainers outside of battles though. It was a stupid choice by Nintendo not to let us fight gym battles again. XP
You must beat the seven gym leaders, the elite four, and your rival. Then you have to capture 60 Pokemon to get your Rainbow pass the will allow you to travel to Island 7 in Pokemon FireRed.
No, unfortunately ;_; The only games that you can Re-Battle Gym Leaders on is Platinum, HeartGold, and SoulSilver
The fourth gym at Pokemon fire red is grass type gym