The 7th gym leader in Pokemon yellow is in Cinnebar Island, which is south of Pallet Town. You need a Pokemon with Surf to get there.
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To get the 8th Gym Leader to return to his Gym in Viridian City, you must defeat all 7 other Gym Leaders.
After u beat the first 7 u will go back to pallet town and then go to viridian and the gym that was earlier locked is now open (Giovanni is the boss)
Beat the first 7 gym leaders then go to the closed gym in Viridian and it will be opened.
You get it by beating Giovanni at the Viridian City gym, which you can't do until you have the other 7 badges.
you dive to get to sootopolis which is in the south east from mosdeep then fight groudon then you can challenge the gym leader and get your 8th gym badge. then u challenge the elite 4.