Where is in soul silver the red Gyarados?

The Red Gyarados is found in Lake of Rage, which is located in the Johto region of Pokémon SoulSilver.

Here's how to get there:

1. Obtain the HM03 (Surf) from the Goldenrod City Game Corner.

2. Head east from Goldenrod City, through the National Park.

3. Use Surf to cross the lake leading to the entrance of the Lake of Rage.

4. Enter the Lake of Rage, and you'll find the Red Gyarados waiting for you.

Note: The Red Gyarados is a very powerful Pokémon and should not be taken lightly. Be sure to bring strong Pokémon with you and be prepared for a tough battle!