Were is the fifth gymleader and how can I reach it?

Beat the other 4 gym leaders and make your way to Celestic Town. After you beat the grunt there, you can fight the 5th leader

I take it you beat the first 4 gyms? Fly back to Pastoria, and go in the Great Marsh and talk to a guy to your right to get HM05 Defog. Exit, and talk to some Galactic Member in the northeast, and chase after him [prepare for a rival battle along the way]. Heal up after the rival battle, and continue chasing him until you battle him. Once you beat him, Cynthia will come up and give you the Secretpotion to give to the Psyduck on Route 210. So, fly to Solaceon Town and go north and give the Psyduck the Secretpotion, and talk to Cynthia again. She'll give you the Old Charm which you'll need to give to her grandmother in Celestic Town. Guess where the northern route of route 210 leads? So, go to Celestic Town using Defog to help you out. Battle the Galactic guy in the middle of the city, and talk to the old lady. You'll give her the Old Charm and she'll ask you to take a look inside the cave. Go in, and examine the wall in the back. She'll come in again and give you HM03 Surf. Now, go back to Hearthome and go to the Contest Hall, and talk to Fantina [the fifth gym leader] in front of the entrance. She'll then go back to the gym.