You have to defeat the other 15 gym leaders, and then talk to Blue (the 16th gym leader) on Cinnabar Island. He will go to Viridian City once you show him the 7 Kanto badges you have obtained.
after you get all 15 badges go to cinnabar island and there will be the 16th gym leader after you talk to him go to the virdian city gym and he will be there
The 16th gyme leader is first on cinnibar island talk to him to make him go back to the viridian city gym.
The 16th gym leader is Blue. You can only fight him after you win all 15 gym badges. after you get all 15, go to cinnabar island, blue will be there. show him your badges and he will return to the gym so you can fight him
First find the gym leader on cinnibar island then go to viridian city you can now enter the gym.
Well after you defeat 15 gym leaders in Pokemon HeartGold / SoulSilver go to Cinnabar Island and talk to Blue.
You must talk to Trainer Blue (Gary) on Cinnabar Island. He will then leave, and you may now go to the 16th gym and battle the gym leader.
go through the foggy route
After beating the 16th Gym leader, you can Catch Mewtwo, and then just train for Red.
After you defeat Blue (the 16th Gym Leader), go out of his gym then, Prof. Oak will call you. Then, go to his lab talk to him and he will tell you that he can make an exception for you to go to Mt. Silver. And now, you can go there.
go right from the seventh gym leader
Beat the previous 14 and then go to Cinnabar Island. Beat the 15th Gym Leader at Seafoam Islands and then talk to the guy that looks like Gary back on Cinnabar Island
Go down to oreburgh mine and talk to the gym leader he will run off now go to oreburgh city and go into gym that gym leader will challenge you.