It depends on what gym you are talking about all of them you can get to by going to the towns.
The 8th gym leader is in Viridian City.
The sixth gym leader,Sabrina, is in Saffron City.
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.
Viridian City, Giovanni(Team Rocket Leader) is the Gym leader.
Step on the portals until you get to the gym leader.
You get it from the Fuschia Gym leader
Yes.
koga fusia city poison gym
In the Kanto region, the sixth gym is in Saffron City. The gym leader is Sabrina.
Sabrina in saffron city
In Fuchisa city
beat the gym leader