This guide shows the location of the 4 Eterna City Gym Trainers that you need to find to be able to battle Gym Trainer Gardenia. The gym is on the southeast side of Eterna City as shown in the picture below. After entering the gym, talk to the leader (Gardenia) and she will tell you that you cannot fight her until you have found and defeated the 4 hidden Gym Trainers. Now go through the door and enter the garden portion of the gym.
Eterna City Gym Trainers Locations
After entering the garden portion of the gym, go straight ahead until you reach the backside with a green patch as shown below. From the green patch, turn around and you will find the trainer standing between some trees.
After defeating Trainer 1, proceed to the southwest side. Go to the end of the path, where you will find the trainer behind some trees as shown below.
Now go back to the green patch. From there go south until you see three trees with light green colored leaves on the right side as shown in the picture below. Trainer 3 is behind these three trees.
Now go back to the garden entrance. From here. take the path on the right side as shown below. Go till the end and then start moving north. At the end of the path, you will see 4 flowers on the left side (See the second picture below). Trainer 4 will be standing near these 4 flowers.
After you have defeated Trainer 4, gym leader Gardenia will appear on the green patch at the backside of the garden. Defeat her to get the forest badge.