Once you fixed the Power Plant, go to the end of Route 25. Talk to Misty and she'll return to the gym. Then you can beat her. (She uses water Pokemon so I suggest you use grass or electric Pokemon.)
Chat with our AI personalities
at the end of the cerulean cape (route 25)
Misty is located around Bill's House at the Cerulean Cape, but you won't find her until you finish solving the problem at the Power Plant.
You best the rest of the Kanto Gym Leaders. Since they are challenged in no particular order, Misty may be your Seventh challenge. If this is so, then you go to Cinnibar Island, where you find Blue (Gary Oak's game counterpart). He is the Viridian Gym Leader, and you take your 8th gym challenge. If Misty isn't your 7th challenge, then you beat the other gym leaders before you can challenge Blue.
beat the 8th gym
you need to beat jasmine in olivine city