technically no, but you wont get anywhere in the game.
No, however, in Pokemon Platinum, it is possible to rebattle gym leaders at the Survival Area.
beat the gym leaders
You have to use a cheat.
you cant.
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.
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.
There is an inn where 3 Gym Leaders battle you (on the Battle Zone)
You can't.
Walk in and save the game then go in and in out
use Pokemon diamond/peral/platinum's effect calculator and find out what the gym leaders weakness is
yeah, i suppose. teach him WATERFALL. using it on the Pokemon is impressive. you will be laughing at the gym leaders.