How do you catch the red gyarados on pokemon diamond?

The Red Gyarados is a standard pokémon you'll encounter in HeartGold/SoulSilver, but not in Diamond. In Diamond it's a 'normal' shiny pokémon, which means that you need to be very lucky to catch one. You can find Gyarados at many places in Diamond, but the chance of getting a shiny pokémon is 0.0122%.