Where is the second gym in Pokemon diamond?

--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.