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 second Pokemon Gym within the Kanto Region is found in Cerulean City. This Pokemon Gym is full of trainers that have Pokemon at Level 16 or higher, and because of this, your trainer will be required to have at least one Pokemon at Level 15 or higher in their Party in order to gain access to the Gym. (Much like the Pewter City Gym, it isn't required to battle with the Pokemon you used to gain access to the Gym while inside of it.)