Unfortunatly you cant, this feature in heartgold/ soulsilver was epic!
You cannot rebattle Gym Leaders.
you cant
you can't!
You cannot.
Hackless, NO!
No.
You cannot.
You cannot re-battle the Gym Leaders in Pokémon Black and White.
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
nope
You cannot fight the Gym Leaders again in Pokémon Black after defeating them and being awarded their Gym Badges.
by winning against gym leaders