Assuming you're referring to the gym in Celadon City, there are lots of things you need to do before you can move on.
Start by going to a restaurant south of the Game Corner to get the coin case. Once you have the coin case you will be able to play in the game corner.
Go to Celadon Mansion and get the Tea to open the divisions to Saffron City.
This will give you access to Saffron City. But you should go there later, now focus on the Game Corner, go to the Rocket guarding the poster and battle him. You must go down the stairs to the basement of the Game Corner which happens to be Team Rocket's Hideout.
You must battle everyone there including Gary and Giovanny, then you will get Silph Cope as a reward.
Once you have the Silph Cope you can go to Pokemon Tower on Lavender Town and go to the top of the tower to earn a Poke Flute.
Once you have the Poke Flute you can go and wake Snorlax to the WEST of Celadon City. From there you will go to the Bike path, on your way to Fuchsia City.
get a life and stop playing Pokemon
i dont know about diamond, pearl, or platinum, but in heart gold and soul silver, you get it by talking to prof oak after getting all of the kanto gym badges.
no you have to beat the gym leader
Olivine City
Train your Bayleaf.
get a life and stop playing Pokemon
the last gym, so just get all the gym badges.
You go to Mt. silver and battle Red.
You have to Have all 16 Gym Badges and Mewtwo is in Cerculaen Cave
Same as every Pokemon game. Defeat the Gym Leaders to get the Badges.
because you need all the gym badges
you have to have all kanto gym badges and talk to oak and he will give it to you!
Battle all the Gym leaders and get another 8 badges. Once you have 16 badges, you should try battling Red at Mt. Silver
You have to get all the gym badges and head east of new bark town
you get it once you have all the johto gym badges then go to Professor Elm and he will give it to you
cinnabar island need 7 kanto badges
Just beat all the gyms. The first one is in Violet City