In order to get Misty and the gain back to the gym. You first have to visit the Power plant way down on north of Lavender Town and under near the Rock Tunnel. (You need to take The Rock Tunnel to get to it) You need a Pokemon that knows FLASH!
Once you are at the power plant talk to the people and they will tell you that a thief stole a piece of the machine. Then go back Cerulean City, go to the gym, the thief will be waiting for you, he will run away, but will be on the bridge. You will battle him but don't worry he has weak Pokemon. After that continue go north, then battle seven trainers to get to Misty. (she will be up on a hill with her Boyfriend)
You can return to the Gym and take on Misty! (Remember she uses Water Pokemon)
Chat with our AI personalities
meet him an island near pallet town
you go back to olivine
You have to look near all the floaties in the Cerulean gym and then once youfind it take it back to the power plant
you need to first go to the power plant and find out that there was a thief and then find him in Misty's gym and then interrupt misty's date on route 25
Go to Blackthorn city. The gym is there