After defeating Blaine of the Cinnabar Island Gym Leader, fly back to Cinnabar Island and Blue should be there. He will explain to you about his Champion life and Red, and he will say that he will meet you at the Viridean City Gym. Fly to Viridean Gym and battle him.
in viridian city
Head to Mt. Silver to battle Red.
The sixteenth gym leader, Blue, should be in Viridan City. To get him to appear in his gym, defeat Blaine in Seafoam Islands (Cinnabar Island's volcano erupted). Then, visit Cinnabar Island to see Blue standing there. After talking to him, he should be at the gym then.
In the gym!
in his gym
fail...
blackthorn
go to the Pokemon gym
move to the 8th gym
He is on Cinnabarr Island.
The fourth gym leader in Pokemon Soul Silver is in Ecruteak City.
get their numbers