1. How do you get to the gym leader because I know where he is but I can..

1. You can start by telling us which gym leader you're talking about!! I think you're talking about the 6th gym, so if I'm wrong, you can't blame me. Sorry, but it's true.
You should know that there are a series of lifts you have to take to get to Byron, that is, if you're talking about the 6th gym leader. Anyway, to get to him, you must battle all trainers along the way. So pass the black belt and the worker and go on the last lift you see, to his right. After that, you go down the only lift possible to take. Pass the ace trainer (I think he's an ace trainer) and go up the only lift possible to take. From here on, you don't have much a choice. Go across using the lift, and go up using the lift you come up to. Take the northern lift from there. Go up until you see a worker, and go on that lift he was guarding. Again, take the only possible lift and and pass the black belt. Go south, and go across using that lift. ONCE AGAIN, take the only lift possible and go down the OTHER only possible lift. Pass another ace trainer and go down that lift. Now, take the red lift, and you're in front of Byron!!

2. Once you've obtained up to 7 badges, and battled Dialga on Mt. Coronet, then you can battle Uxie and Axelf. Once you try to battle Mesprit, he'll run off in the wild. You can use the marking map to locate him.

1.Which Gym Leader?
2.You didn't list whom, so I'm guessing you mean Mesprit, Uxie, and Azelf(pwnage). These three can only appear after finding Dialga/Palkia. Mesprit is at Lake Verity(near Twinleaf Town), Uxie is at Lake Acuity(near Snowpoint City), and Azelf is at Lake Valor(near Pastoria City).

Not to double post, but I left out something. When you find Mesprit at Lake Verity, she'll run away. You can track her however, using the Marking Map Poketch application.