You'll need to conquer all of Pokémon Brilliant Diamond and Shiny Pearl's Gym Leaders to progress through each match, but it's not like they just disappear once you defeat them. In fact, they are quite a large part of the post-match, as you can challenge them to rematches that are sure to test your skills as a Pokémon trainer. Here's how to match Gym Leaders in newly released Generation IV remakes.
Like many of Shining Diamond and Shining Pearl's post-game activities, you'll first need to complete the game's story and unlock the National Pokédex, which requires you to see all Pokémon in the Sinnoh region. After acquiring the National Pokédex, Oreburgh City Gym Leader Roark will appear and declare that the Gym Leaders would like to face you again. From here, you can rename each Gym Leader once a day by simply talking to them in their gyms.
Each Gym Leader will have a much stronger team this time around, with most of their Pokémon hovering around level 70. Additionally, each Gym Leader will have a well-stocked team of six Pokémon, each with a held item. If you face the Gym Leaders again, make sure your Pokémon are at least level 70; we recommend grinding at level 75 just to be safe. These battles are no joke and represent a very daunting post-match challenge.