The seventh gym key is in the Pokemon mansion. Hope that helps good luck.
The 7th leader is Blaine. He uses fire pokemon.
Go to pallet town where the water is, then surf down.
You will need the Secret Key which can be found in Pokemon Mansion on Cinnibar Island.
Fight the seventh gym on Cinnabar Island. Then fight the eighth gym at Virirdian City.
the seventh gym is at sekka city
go to cicibar island, then go in the biulding right next to the gym and look for secret key. then get out and then you can go in the gym in cinabar island
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)
get the key in the od mansion on cinabargo to cinnibar island, then go in that place next to the gym and look for secret key. then get out and then you can go in the gym in cinabar island