Where do you find Red Gyarados in Pokemon X/Y?
Solution 1:
The red gyarados isn't unique- it's simply a shiny gyarados. There isn't a specific event for it in Y (or in any games except gen 2 and the remakes) but any way of getting a shiny pokemon will work here. Catch a shiny gyarados or evolve a shiny magikarp.
You can also transfer a red gyarados caught in Heart Gold/Soul Silver to Y, if you own one of those games.