Pleaselog inorregisterto add a comment.
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.
Source
You don´t exactly catch a red gyarados it´s a shiny Gyarados and you must either Breed for one or Try to find 1 at the wild but it´s a 8192 to find one so it´s pretty hard unlike sould silver / heart gold.