The fifth Gym can be found in Fuchsia City. The leader is Koga, who specialises with Poison-type Pokémon.
In Celadon City, your rival is Erika.
The fourth gym at Pokemon fire red is grass type gym
You cannot rebattle Gym Leaders.
fire
after you beat the fire gym then walk out and your rival will give it to you.
charizard
In Celadon City, your rival is Erika.
You go to saffron city
Koga's Pokemon are: Koffing Muk Koffing Weezing
no
In Saffron City, the top right there are two gym looking buildings. The one on the right, I believe is the gym.
it is in saffron
In his Gym in Cinnabar.
the thunder gym in in vermillion city
You have to beat the 2nd gym.
In the cinnibar island gym.
beat the gym leader