To be able to fight Gardenia in Evisopolis, Pokémon Shining Diamond and Shining Pearl's second gym, you'll first need to track down the other four trainers hiding in the room. While they're not extremely hard to find, they will only appear in a specific order, which can make it a little more awkward.
Below, you'll find details on where to find all the trainers, along with some screenshots showing their exact locations.
First Coach
The first trainer can be found by walking all the way down the middle lane, then turning left when you reach the end near the white fences. She is standing right in front of the tree in the empty space.
Second Coach
From the first position of the coach, walk north, then left to cross another gap. He goes all the way down past the two rocks and turns right to find the trainer.
Third Coach
For the third manager, go back to where you found the first manager and go to the middle lane. Go down over the rock and you will see three different looking trees on the right. The third coach is behind them.
Fourth Coach
The final coach is located in the upper right corner. He walks to the door you entered the room from, 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 upper center of the room to find Gardenia. Gardenia will use Cherubi at level 19, Turtwig at level 19, and Roserade at level 22.
