You will need to go to Cerulean City next which you can reach by going through Mt Moon on Route 4 which is located east of Pewter City.
after you beat the fire gym then walk out and your rival will give it to you.
Simply go inside.
You go to saffron city
Simply go inside.
you go in the Pokemon mansion and the gyms key is some ware in there.
go to cinnabar island then go into Pokemon mansion go through here to get the secret key which opens the 7th gym.
The 8th Gym in Pokémon FireRed and LeafGreen is actually in one of the first towns you go to. The gym is located in Viridian City.
Go to the Pokemon League, West of Virdian City
You need to go to the Pokemon mansion in Cinnabar Island. Go to the basement and obtain the Secret Key. Then go to the Pokemon Gym and it will be unlocked!
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
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
get Pokemon to level 20 then go to the Pokemon center heal your Pokemon the it will learn cut after you defeat the second gym leader