Thanks Chris, Hasty was my top choice until i second guessed myself.I remember Gyarados can only get the happy hour move (not celebrate but could be wrong) anyway i have seen in a past a z-happy hour moveset with hasty nature, waterfall, earthquake and special move flamethrower...
Interestingly, Gyarados gets Celebrate AND Happy Hour via the Magikarp distributed in this event. This is the only case that i am aware of where an event 'mon gets more than one variant of Celebrate from a single event.
What the Bulbapedia page doesn't tell you is that it can also be shiny at full odds, as the PID type is random rather than fixed as it is for most other events. To get the Nature I want on a shiny Magikarp will mean sacrifices will need to be made for IVs - odds are too high to get a flawless one.