How to Easily Get the Red Gyarados in "Pokemon Ruby"by Amanda Rumble ; Updated September 22, 2017
The Pokemon Red Gyarados is an unusual Gyarados that is shiny, unlike the typical blue dragon-like Pokemon. It is red because it evolved so rapidly from Magikarp that it didn't have time to change colors. The chances of getting a shiny Gyarados, regardless of what game you are in, including the Game Boy Advance game "Pokemon Ruby," are one out of 8,192.
Go to Sootopolis City.
Fish in the lake for a Gyarados.
Continue fishing in the lake until you get a shiny, red one. If you are persistent, you will catch one eventually.