In order to find the seventh gym you need to get to Cinnibar Island. In order to do that you must beat the fifth gym leader. After or before you beat the fifth gym leader ( I prefer before ) you need to go into the safari zone and find the HM surf. Surf allows you to go on water. Surf is at the very end of the safari zone in the last house. Talk to the person and he will give you surf. Once you can use surf ( after beating the fifth gym leader ) go down and to the left. You will fight about 10 trainers and enter Seafoam Islands. The way to explain how to get past seafoam islands is difficult. Just try to figure it out yourself. It will give you a good challenge. :) :) :) I hoped I helped!!
fire
The 7th leader is Blaine. He uses fire pokemon.
The basement.
If you surf south of Pallet town you will get to Cinnibar Island. It is on there.
Fight the seventh gym on Cinnabar Island. Then fight the eighth gym at Virirdian City.
u can find it in Pokemon mansion.
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
In the 4th gym ! After you beaten the gym leader in saffron city!!
I beat it a long time ago so I'm not sure but I think you have to find the key to the gym in the basement of the poke mansion. or maybe is the boss from team Roket...the gym is in ....in ..remember the locked gym on the first town you get to take Oaks Parcel? im sure you do
He is in the Cinnabar Gym on Cinnabar Island.
viridian city
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)