the seventh gym leader can be found at cinnabar island which is accesible by getting surf (complete safari zone) and going south of fuchsia city or by going to pallet town and Surfing in the water south until you get to a giant mansion jump on the island and head into the mansion or go to the Pokemon center now you need to run around the mansion and eventually you end in the basement, nab the key use an escape rope and head to the gym, now after you defeat blaine you get the 2nd powerful fire move in the game :D
On cinnibar island.
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
The fifth Gym Leader in Pokemon LeafGreen is Koga, the leader of Fuschia City's Gym. He specializes in Poison-type Pokemon and hands out the Soul Badge.
The 8th gym leader is in Viridian City.
In the mahogany gym
The seventh gym leader is Candice, the Ice Type leader. She is at Snowpoint City.
The key is in the Pokemon mansion.
Viridian City, Giovanni(Team Rocket Leader) is the Gym leader.
Step on the portals until you get to the gym leader.
You go on to defeat the eight gym leader.
In Fuchisa city
beat the gym leader