you cant.
No, however, in Pokemon Platinum, it is possible to rebattle gym leaders at the Survival Area.
beat the gym leaders
No, but you can challenge kanto gyms
You have to use a cheat.
I don't think you can re challenge a gym leader you have already beaten in Pokemon white.
... No
You go to Sunyshore and challenge their gym.
the gym leaders are the same as they are on pearl and diamond. but the order you fight the gym leaders are different. Turtwig would be the easiest way through the game.
technically no, but you wont get anywhere in the game.
no you can't
Go in to every Pokemon gym and get beaten at least 10 times againgst the gym leaders.
You'll have to go to the gym and fight with them with your own equal or better Pokemon team.