Pokemon Diamond Version

Unfortunately, you cannot rematch with any Gym Leaders in either version of this game. The VS seeker, however, allows you to rechallenge trainers you've already fought -- that is, if they're up for it. The determination of whether their up for a match is decided using a random number generator, with odds based off how many battles you've had from the first fight with the trainer. It seems the less battles the better, and this DOES count wild pokemon -- even if you do just run away from it.

nope, it would be nice if we could but unfortunately, they cut it out of this game for reasons they had during development. And it's also very uncertain if it will appear in pokemon platinum. All i'm saying tho is that while it's nice with all the extra features, there are still things that can be debatable and yet still personally liked

Nope. You can use the V.S seeker to get rematches with other Trainers, but nobody from Gyms.

No, but the VS. Seeker lets you challenge trainers that aren't in a gym, though.

Unfortunatly, you cannot re-battle Gym Leaders in this game. Why they took this feature out is beyond me, it was quite fun. You can re-battle Trainers around the region though. You can walk up and talk to them to check, which will start the battle automatically if they will battle, or use the VS Seeker to check first.

You can't rematch with Gym Leaders after the Elite 4, but the Vs. Seeker allows you to rematch with trainers.