From Bulbapedia, the community-driven Pokémon encyclopedia.
The Eterna Gym (Japanese: ハクタイジム Hakutai Gym) is the official Gym of Eterna City. It is based on Grass-type Pokémon. The Gym Leader is Gardenia. Trainers who defeat her receive the Forest Badge.
In the games
The Eterna Gym consists of a green foyer, past which is a greenhouse room where an indoor forest is arranged in a maze-like pattern. The Trainers of the Gym hide and wait behind trees and natural surroundings, and Gardenia will not accept challenges until all of her students are defeated too. Every time a Trainer is defeated, they give a hint about where another foe is. Trainers of the Gym are fond of using status-inducing moves such as Poison Sting, Leech Seed, and Stun Spore.
In Pokémon Platinum, the entire Gym has been redesigned. The Gym now contains four areas with a giant clock in the middle, separating these areas. For every Trainer the player defeats, the hands of the clock move in a certain direction. Once the clock's hands spin to the north, south, east or west direction, the player can walk onto them and progress to other areas of the Gym. A set of sprinklers also turns off, allowing the player to exit the Gym and return later. Gardenia awaits the player in the northern area of the Gym (when the larger hand reaches 12 o'clock).
In the anime
The Eterna Gym first appeared in the anime in the episode The Grass Menagerie! when Ash challenged the Gym. In the anime the Eterna Gym is modeled like a large indoor garden, complete with trees and grass. It has been shown that many types of Grass-type Pokémon live there being taken care of by Gardenia and her helpers. Unlike the other Gyms it doesn't have an official battle field for it; instead, they battle in a large clearing in the middle of the Gym.