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.
Go to pallet town, and go south to cinnabar island. you will find that the only thing there is a Pokemon centre and a guy. Go east and you will find an island with a cave. Inside this cave is a gym (go up the ladder inside). After beating this gym, fly back to cinnabar and talk to the guy. He will say hes the gym leader of viridian city. Fly there, beat him and you will have all 16 gym badges.
In Johto you have to travel back to goldenrod city and go into the underground tunnel and go towards where the camera man is usually stationed. The Team Rocket member down there will think you are a new recruit and will give you the Team Rocket uniform. After you get the Uniform go to the radio tower and work your way up to save the Manager person. Once you finish the radio tower go back to Mahogany Town and go past the guy that wanted you to buy the RageRareCandy Bar. From there you can go into the Ice Cavern and make your way to Blackthorn city.
In Kanto, you have to go to Cinnabar Island and talk to Blue after you have 7 gym badges. He will then think you are a worthy opponent and he will go back to his gym in Viridian City.
Hope it helps!
After defeating Pryce, you'll recieve a phone call from Elm once you leave the Gym. Head to Goldenrod City and into the underground tunnel. On the northern side, there's a rocket grunt who'll give you a uniform to sneak past a guard in the Radio Tower. You won't be able to defeat Team Rocket unless you get that uniform.
In Pokémon HeartGold, after beating Falkner in Violet City, go talk to Professor Elm's aide in the PokéMart in the city and the aide will give you the Egg you had picked up from Mr. Pokémon before. Now continue on your way to Route 32 and head south until you enter Union Cave. Follow the path there and you'll wind up in Azalea Town.
After getting the 4th Gym Badge in Pokémon HeartGold and SoulSilver, your next course of action would be to head to Olivine City and go up the stairs to the top room (aka The Light Room) of the Lighthouse and talk to Jasmine there. She'll tell you that Ampharos, the Lighthouse's Light Pokémon is sick and that it needs medicine. She'll tell you that there's a Pharmacy in Cianwood City and she'll ask for your assistance. Head to Cianwood City, go into the Pharmacy to get the SecretPotion and then challenge Chuck the Gym Leader there.
get the 15th gym badge, talk to gary/blue on cinnabar island, and then get the 16th in viridian.
go to goldenrod to fight the rockets and get the silver wing
You go to Azalea town for your next gym badge.
i don't know i don't do pokemon (-_-)
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