--ORIGINAL ANSWER--
Etrana city, upwards from Floaroma town through etrana forest and whoop!
--IMPROVED ANSWER--
It is in Etrana City. Head west from Floaroma city, you will be need solve the problem at the windworks before you can go across the bridge. After you cross the bridge, continue north, up through Etrana Forest, and go west. You will then be there.
You can find the 2nd gym in the south part of Eterna City.
The second gym leader in Eterna city
The first gym in Pokemon diamond is in Oreburgh City
where can i find the second gym in Pokemon platinum
the sixth gym is in canalave city
The Pokémon the second Gym Leader has in "Diamond" are Cherubi, Turtwig and Roserade.
eterna city
Bottom right of Eterna City
You can find the 2nd gym in the south part of Eterna City.
Eterna city at bottom right
The second gym leader in Eterna city
After you do the second to last gym it becomes available
Are you serious? Washroom!
the fifth gym on Pokemon diamond is the one in herthome city,
The first gym in Pokemon diamond is in Oreburgh City
eterna city gym leader gardenia she uses grass types
Use a fire pokemon. That's what grass is weak against.