Question: what to do after beating 7th gym in soulsilver?
Answer: Go to the radio tower and run off team rocket. ... 
Fight off team rocket in the radio tower in goldenrod. ... 
Go up to the Lake of Rage, fight and KO/capture the Red Gyarados in the center-ish of the lake, Surf back down to the shore, and a certain someone will be waiting for you to further the plot.