It is on Mossdeep City. There are Twin Gym leaders
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talk to Wallace
to get the eight badge go to viridian city after you have 7 badges and enter the gym
7
7
You will get the 7th Badge which is the Volcano Badge from Blaine, the Gym Leader of Cinnabar Island however you will need to receive the Key from within the Pokémon Mansion before you can unlock the Gym's locked door.