You don't that's only for hh & ss (heatgold and soulsilver)
You cannot.
Hackless, NO!
No.
You cannot.
You cannot re-battle the Gym Leaders in Pokémon Black and White.
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
Yes, there will be Gym Leaders in Black and White. It has the usual 8 Gyms however there are 2 differences. The first Gym has 3 Gym Leaders but you'll only fight 1 of them depending on the Starter you chose and the last Gym has 2 Gym Leaders but the Gym Leader for that Gym differs depending on your game.
no only once
you have to beat gym leaders
After beating all the Gym Leaders, complete your pokedex.