In Veilstone City
Actually there is no gym leader in celestic town. After you beat pastoria gym go to hearthome city.
you need to defeat pastoria city's gym leader
The first gym in Pokemon diamond is in Oreburgh City
There is no battlable character named Katrina in Pokemon Diamond. However, the Gym Leader Fantina can be battled in her gym after delivering the Charm to the grandmother in Celestic Town
The Pokémon the second Gym Leader has in "Diamond" are Cherubi, Turtwig and Roserade.
Beat Maylene then, Wake. (Crasher Wake). Then,go to Celestic town. Then, battle her.
The 6th gym leader is in Canalave City.
Use surf.
you beat the gym leader
He is in the lighthouse
It is Candace, The Ice-Type Gym Leader.
Go to the lighthouse and the Gym leader should be there.