--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.
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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