You must have HM05 Flash to go through Rock Tunnel. Rock Tunnel is to the east of Curlean City. You can get HM05 by going through diglett's cave(and end up near pewter city) and going south to a buiding to which you will find Pro. Oak's aid. If you own 10 pokemon, he will award you with HM05.
After you beat the gym, you must leave, heal ur pokemon, then go back and talk to the leader and she should give you the badge
lol your a noob
I'm pretty sure you go to Celadon City to try and beat Erica - or was THAT the 3rd Gym Leader?
If you are talking about after the gym is complete, you go back to Viridian City to the (formerly) locked gym. You will get a nasty surprise, but I will not spoil it.
You have to go into Pokemon mansion. Find the key in the basement.
go to the Pokemon gym
Lt. Surge is in Vermillion City. Go to the S.S. Anne get cut and cut the tree in front of the gym
to veilstone city
You have to skip the thied and go to the 4th then go to the third.
Go beat the fourth
just go to mauville city and you'll find the gym
You must go to the Pokemon Gym. Beat the gym leader and he awards you with the 8th gym badge
mauville city
The third Gym is the Gym in Veilstone City. You can get there by looking on your map. The leader specializes in Fighting Pokemon.
you go to the Pokemon mansion next to it and at the very end of it is the key to the gym
go to the third gym (lt.surge)
The third gym in Pokemon diamond is in veilstone city. The gym is a fighting type gym so flying types are super effective on the fighting type Pokemon. Fire type moves are also effective on her lucario. Ghost type moves are also effective on her meditite. To get to the gym you have to go south from the Pokemon center after you beat team galactic in veilstone city. Then go west until you reach the farthest set of stairs then go north to the gym.