How do you get to the gym leader in fortree city?

Answer 1

There's an invisible Kecleon blocking the way to the gym. Just continue east on to the next route and on a bridge Steven Stone will cruise in and give you a thingy that'll reveal them. Then you'll have to battle/catch it. They're typically level 30 and change their defensive type to the last move used on them. To beat them quickly, use a Pokemon that has two moves that'll do serious damage to one another, such as a fighting move and then a flying move (like with your Combusken/Blaziken (you kept peck, right?))