i suggest you start by trying all the warp panels.one of them will lead you to sabrina.
the 5th gym is saffron gym with sabrina!
Koga, he is in fuschia city, he uses poison Pokemon
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
pewter city. go through the viridian forest
You have to go through the game until you get fly.
you must defeat the 5th gym and get surf in safaria zone the go to Pallet Town and surf downwards.
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 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.
go to saffron city to beat Sabrina to get the 6th badge however you need to first go through silph co to do this.