You must go and battle Misty. Once you defeat her, she will give you a badge. You have then defeated the gym.
go to diglett cave
Kanto region north of saffron city. Next to the Pokemon center
In Pokémon HeartGold, you can gain entry into Cerulean Cave once you've defeated Blue.
Not Heartgold right? But the old Pokemon Gold? The thief is in the gym in Cerulean City.
First you need to return the machine part to the power plant then find the gym leader on route 25 which is northeast from cerulean city.
beat the power plant part its in the cerulean gym sorry for spelling mistakes
Go to the Cerulean gym. Press A next to one on the piles of innertubes. On the west side of the gym i think.
He hides the generator in the Cerulean Gym beside a swimming tube at the back of the gym (Around where misty would have been in the Gym)
Go to the Cerulean City Gym!
After you beat the Cerulean Gym it is up on the cape where you find Misty. You have to see it at other points during the game though
It is in the Cerulean City gym. Click on one of the inner tubes and you will obtain the Machine Part.
The machine is hidden in the Cerulean Gym