Things You'll Need
Instructions
Insert the Pro Action Replay device into your GameBoy Advance, and turn the power on. Go to the entry for "Pokemon LeafGreen."
Enter a new code titled "Shiny Pokemon." On the first line, input "72BC6DFB" and on the second, enter "E9CA5465." On the third and fourth lines, enter "56671F3A" and "6F4F4D6B." Start the game.
Walk to the shore near any body of water and use your fishing rod to fish there. If you catch a Gyarados, you enter a battle with it. In the battle, the Gyarados is shiny and red.
Weaken the Gyarados until its health bar turns red. Throw Poke Balls at it until it is captured and you now have a shiny Gyarados.
Any Pokemon you encounter in the wild, as long as this code is on, will also be shiny.