hearthome city leader
water
The first gym in Pokemon diamond is in Oreburgh City
The Pokémon the second Gym Leader has in "Diamond" are Cherubi, Turtwig and Roserade.
The 6th gym leader is in Canalave City.
you beat the gym leader
He is in the lighthouse
The water Pokemon gym leader is in Pastoria City.
It is Candace, The Ice-Type Gym Leader.
Go to the lighthouse and the Gym leader should be there.
by winning a contest and by defeating the pastoria gym leader
just beat the fourth gym leader
a gym leader can't teach a Pokemon that move but you have to defeat the 6 gym leader in order to teach that move