its in sunnyshore city, but you have to catch dialga first. same with pearl but you catch palkia.
Sunnyshore City Gym: Electric (beacon badge from Volkner)
The 8th Gym Leader has electric type pokemon. Your best bet to defeat him is Ground or Rock pokemon. Electric types are not effective against ground types.
Volkner, he will be at the lighthouse, 2nd floor. Talk to him and he will go to the gym waiting. Uses electric types.
The first gym in Pokemon diamond is in Oreburgh City
the fifth gym on Pokemon diamond is the one in herthome city,
you get the HM Flash after beating the electric gym leader, if i recall correctly
all you need is a ground type, but for octillery, maybe use an electric Pokemon or grass one, or it will slaughter your ground one otherwise
He is at the lighthouse. Bring ground types and stuff because he is the electric leader!
The seventh gym in Pokemon Diamond, Pearl, and Platinum is located in Snowpoint City, which is to the East of Lake Acuity.
The Pokémon the second Gym Leader has in "Diamond" are Cherubi, Turtwig and Roserade.
Wattson the electric gym.
hearthome gym. you know where that is.