How to Get Red Gyarados in the "Pokemon Sapphire" Versionby Mitch Reid ; Updated September 22, 2017
In “Pokemon Sapphire,” Gyarados is Water and Flying-type Pokemon with a fierce temperament. Its aggressive nature complements its ability to learn powerful attacks such as Dragon Rage, Twister and Hyper Beam. Like all Pokemon species, Gyarados occasionally come in different colors. This typically blue Pokemon can also appear with red scales. While it's rare in most Pokemon games, it can found in a lake in "Sapphire."
Travel to Mossdeep Town east of Route 124 in eastern Hoenn.
Visit the house on the hill on the eastern side of the island.
Talk to the fisherman inside to receive the Super Rod item.
Go west on Route 126 and use "Dive" in any dark water. You will enter an underwater passage that leads to Sootopolis City.
Fish in the lake in the southern section of Sootoplis City until you hook a red Gyarados or a gold Magikarp. A gold Magikarp evolves into a red Gyarados at level 20.
Throw a PokeBall to capture the red Gyarados.