Well the Gyarados will always be shiny, so you don't have to use one of your shiny seeds. Also, it's generated by "Chained Shiny Theoretical", not Method K.
So what I did was found a Method K seed, pasted it into RNG Reporter, and changed the Method to Chained Shiny. Then I looked to see if the nature / IVs I wanted were there.
After I found a seed I saved in front of the Gyara, hit my seed, and advanced my frame. It's basically like any other stationary abuse, except if Gyarados moves the frame advances (But it's easy to not let him move if you pause the game right away). Any questions or do you think you got it? :)