If you are referring to the first gym, in Viridian, you will have to wait until you have completed all the other gyms. There is a guide in the related links below.
The key to the 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.
the 5th gym is saffron gym with sabrina!
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
I think the gym in fuschia city
viridian city
there isn't one
Vermilion city.
The 6th gym is in Saffron City the gym leader is Sabrina she uses Psychic type pokemon.
in fuchsia city