All you have to do is get to Viridian City.
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Blue (The eighth gym leader) is waiting on Cinnabar. He won't battle you unless you've beat the rest of the gym leaders
Yes, to get to the Electricity Gym
You beat the 6th gym leader.
You have to show Blue the other seven Kanto badges so the guy in front of the Viridian City gym will move out of the way. Blue is on Cinnabar Island.
Go to Cinnabar island and talk to Blue. He'll agree to go back to his gym and battle you.