Actually there is no gym leader in celestic town. After you beat pastoria gym go to hearthome city.
Chat with our AI personalities
Take of the phyduck
Celestic town shrine
You can find it near celestic town in route 210 or 211.
You have to go through route 210,then Solaceon city.
Beat a Gym Leader, Cynthia talks to you, tells you to visit her grandma in Celestic town, and gives you a Secret Potion. Use it on the Psyducks and you can get past them.