To be able to battle Gardenia in Eterna City, the second gym in Pokémon Brilliant Diamond and Shining Pearl, you will first need to track down the four other trainers that are hidden around the room. While they are not hugely difficult to find, they will only appear in a specific order, which can make it a little more awkward.
Below, you will find detail on where to find all of the trainers, along with some screenshots showing their exact locations.
The first trainer can be found by walking all the way up the central lane, then turning left when you reach the end near the white fences. She is standing just in front of the tree in the gap.
From the first trainer location, walk north, then left to get through another gap. Go all the way down past the two rocks and turn right to find the trainer.
For the third trainer, go back to where you found the first trainer and go into the center lane. Walk down past the rock and you will see three different looks trees to the right. The third trainer is behind them.
The final trainer can be found in the top right corner. Just walk to the door you entered the room through, then go right and follow the path all the way around.
When you have found all four trainers, return to the fenced area in the top center of the room to find Gardenia. Gardenia will use Cherubi at Level 19, Turtwig at Level 19, and Roserade at Level 22. Make sure you read up on in-depth tactics for the Gardenia battle. Once you have finished the fight, make sure to get yourself a bike to make traveling much faster.