>>39534211It seems he's the first you face, but he may be the second gym you go to.
The latest info made it clear that the opening ceremony for the League takes place in the stadium of the Red City. You can see the chimneys around the outer edge of it here.
So we start by doing tutorial town stuff, then go through the Wild Area to the Red City, the opening ceremony happens, then most likely we're sent off and aren't able to fight the Red City gym leader (presumably Kabu mentioned in the leak) until later. So we go to Milo first.
Would explain why his Pokemon are so high level for a first gym, we have to go through a fuckload more than usual to reach him.