Red Gyarados must be found in Pokemon Diamond the same way as you would find any other shiny Pokemon.
You are probably thinking of the Red Gyarados in the gen 2 games and their remakes, which can be found in the Lake of Rage. In any region besides Johto a shiny Gyarados can only be obtained through evolving a shiny Magikarp, finding a wild shiny Gyarados, trading, or transferring.
Good luck trying to find a shiny Magikarp because they are 1/8192.
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