Unlike previous Mainstream Pokemon Games, every Pokemon Gym found within this game have certain requirements that must be met before your trainer will be allowed to challenge said Gym. The seventh Pokemon Gym within the Kanto Region is found on Cinnabar Island. This Pokemon Gym will initially be locked when you first arrive on the Cinnabar Island, and because of this, you will be required to explore the Pokemon Mansion also found on the island in order to retrieve the Secret Key. With this key, you will be able to unlock this Pokemon Gym. No further requirement is needed from there.