Use a lot of rock- and ground- type Pokemon, they're super-effective on electric-type.
grass and electric types
You have to go into the pokemon mansion, find your way through it, and pull a switch and the gym will open.
go through victory road with all gym badges
You have to go through the game until you get fly.
pewter city. go through the viridian forest
The key to the gym
You need to go through the Pokemon mansion and beat all the trainers. At the last one you get the "secret key" to Blaine's gym.
the 5th gym is saffron gym with sabrina!
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.
Yes, if you get the Gym Badges.
Sabrina's gym in saffron city
there is a key in the Pokemon mansion... you need it to get into the gym... search the Pokemon mansion thoroughly