On the island where the 7th gym is at there's a mansion.
go in and make your way to the highest floor you should find the key there.
There will be wild Pokemon and trainers to battle there also.
Hope this helps!
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On cinnibar island.
The key is in the Pokemon mansion.
you must defeat the 5th gym and get surf in safaria zone the go to Pallet Town and surf downwards.
in the mansion.
its in cinnabar island, south of pallet town use surf to get there it is a fire gym
The seventh gym is on Cinnabar island,right down of pallet town.To open the doors you must reach to a certain room of the Pokemon mansion.You get a key and open the seventh gyms doors,and the gym is a fire gym
the seventh gym is the first gym at the begining of the game. the leader is giovani of team rocket. he's ground and normal type
If you mean Pokemon FireRed/LeafGreen on the GBA, the mansion is in the top left corner of Cinnabar Island. (home of the seventh gym)
You will need to use the Secret Key which is hidden in the basement of Pokemon Mansion which is located on Cinnibar Island.
the seventh gym is at sekka city
The key to the gym
the 5th gym is saffron gym with sabrina!