Beat the Elite 4, after which construction on the ferry will be completed and you'll be able to go to the Battle Tower (Ruby and Sapphire)/Battle Frontier (Emerald).
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In Slateport and in Lilycove
When you learn to use the force.
Slateport City doesn't have a gym.
beat elite four then go to ferry port in lillycove or slateport and it will be there. Hope this helps!
take the ferry from lilycove or slateport and Scott will be on the ferry and tell you thenext ferry you take is going to the battle frontier.