Pokemon Diamond Version

Red Gyarados?

Is it possible to find a Red Gyarados in Diamond and if so how? Or is it just in Pokemon Silver?

it is super rare, but, it is possible to find shinies(i know its possible by my friends shiny volbeat from emerald)
unfortunately, there is no special point in the game. perhaps nintendo is remaking gold/silver. (besides the fact that there is a report about red gyrados, 2 more clues are that this one guy who gives you happiny egg says proffeser elm of johto talked about eggs. another clue is... well i can't remember her name, but I think its jasmine, who is in sunyshore.

I played non stop for 8 hours and i found one in lake valor

I have a red Gyarados. You have about 15% chance of finding one. BUT there are no special occasions where you can get them.

You have to fish with the super rod and if your lucky you will find it.

There will likely be a remake of gold/silver/crystal. If so there will likely be a Red Gyrados in the game(s). Wait for this, use a cheating device, or be a very patient fisherman.