You go through Eterna Forest. When you reach too the end, a person should be with you and say 'Bye' then walks off. When you get out of the forest, you should see a bridge with fisherman on it. Avoid them (if you want, battle them) and you should reach to Eterna city, where the second gym should be there.
The Pokémon the second Gym Leader has in "Diamond" are Cherubi, Turtwig and Roserade.
eterna city
Eterna city at bottom right
The second gym leader in Eterna city
i think the 2nd gym leader is in Jublife or Oreburgh city.
The first gym in Pokemon diamond is in Oreburgh City
Use a fire pokemon. That's what grass is weak against.
eterna city gym leader gardenia she uses grass types
The Second Leader Is In Eterna City
The 6th gym leader is in Canalave City.
you beat the gym leader
He is in the lighthouse