Were you looking for the Sinnoh Battle Frontier?
The Battle Frontier is an island located in Hoenn. Pokémon trainers come here to enter various Battle Buildings and defeat their leaders to earn a Frontier Emblem.
In Gradient Hue
In Gradient Hue, the Battle Frontier is considered the games HUB. When Gradient and his friends first enter it, they are trained by the leaders of the Battle Buildings. He is then awarded Bladion. After this, several wormholes open up around them to various other worlds and Gradient goes to them to find the Iguana Battalion and defeat it. Gradient comes back here several times and is also able to battle the Battle Building's leaders and their Pokémon for practice. At the end of the game, when the Final Levels start, the connections to other worlds become thin, and a major wormhole appears offering access to the Final Levels and Gradient's ultimate destiny.