I love how people are mentioning that this guy's Water and Flying STABs provide good coverage. Now, before anyone else says this, ask yourself: Aside from Hidden Power Flying and Bounce, what Flying Type attacks can Gyarados learn?
Yeah... I try to type my typing reviews as pokemon neutral so I can recycle the type when it comes up again. Gyarados' is so limited in its secondary STAB that its funny. Frankly, in my opinion, bounce is arguably the best of the 2-turn moves, because aside from Ferrothorn (which is admittedly a common switch) the paralysis chance on Bounce screws even switch ins that resist it.
I was once routed by a Gengar who critted 4 times in a row on my team. Morale of the story; Crits happen and Sucker Punch is good Gengar repellent.