That is the Saffron City Gym. You have to get the Tea from an old lady in Celadon, give it to the guards around Saffron, and they'll let you through. You then have to beat team rocket in the tower and then you can take on the gym.
In the abandoned house on the right of cinnabar island. You need to figure out an puzzle and then you find the key to the gym.
They are generally in their gyms. If you are more specific (include name in question) you'll be able to get a more specific response. Find below a pokemon walkthrough that should help
In the 4th gym ! After you beaten the gym leader in saffron city!!
no
Giovanni
saffron city (psychic type gym leader)
The 7th leader is Blaine. He uses fire pokemon.
In the 4th gym ! After you beaten the gym leader in saffron city!!
no
The Gym Leader that allows you to use the HM move Strength in Pokemon FireRed is Erika. Erika is the Gym Leader of Celadon City and is the 4th Gym Leader the player faces.
beat the gym leader
Giovanni
saffron city (psychic type gym leader)
there is no gym leader in lavender town
charizard
26
The 7th leader is Blaine. He uses fire pokemon.
You cannot be a gym leader yourself in any of the Pokemon games, so far.
Gym 5 is located in Safaron City, and the Gym Leader is Sabrina. She uses Psychic type Pokemon, and it's a bit of a puzzel to get to her.