Did the Gyarados become red? If it didn't, relogging would help. If you can prove you bought it as a shiny to an admin in the discord, making a ticket prob could compensate the mon back(I hope that for you!)
You'll need to make a ticket within our Discord server for us to look into this as this isn't a common issue we are aware off. My only idea at the moment would be it was a skinned magikarp that does not have a skin for Gyarados but again even that is unlikely.