Pokemon Brilliant Diamond & Shining Pearl is a remake of 2006 NDS and is a role-playing game developed by ILCA. Once you defeat the first gym leader you will continue your adventure towards Eterna City where you need to battle the second gym leader to get the forest badge. After entering the gym you will interact with the gym leader at the entrance who will only battle with you once you find and defeat all the gym trainers hidden in the gym forest.
There will be a total of four gym trainers that you need to find, battle, and defeat for the gym leader to spawn. In the beginning, all the gym trainers will be hidden that can be hard to locate and you need to find the first one, and then only the remaining gym trainers will spawn. Below you will find the location of all the gym trainers and the pokemon they will use. If you have chosen Chimchar as your starter pokemon you can easily defeat the gym leader and obtain your 2nd badge.
Pokemon Brilliant Diamond & Shining Pearl
To find the first gym trainer you need to go straight to the end of the room and look for a pathway on the left side. Once you enter from the pathway you will find the gym trainer. Check out the above image to find the exact location.
From the first trainer, go towards the extreme left, then head bottom and on the right side, you will find the second trainer.
Head back to the middle of the room and on the right side, you will find three different-looking trees. Behind the tree will be the third gym trainer.
Now come back to the entrance of the gym and go towards the right side, then head towards the top, and on the left side, you will find the final hidden trainer that you need to defeat for the gym leader to spawn.