You can't rebattle the gym leaders but you cna rebattle the Elite Four. They will be stronger the second time.
Now I also have a question myself: When I trade through the GTS Negotiation, I get to see how many times a person has traded (I think) before I trade with them. How do I figure out how many times I myself have traded?
Also is there any way to view some of your gaming statistics? In HG/SS for example, there was a computer in the Pokémon Center you enter before entering the league where you could see how many Pokémon you have battled, caught and hatched. Is there a thing like that in B/W?