Seriously, Gyarados flying types added some good resistances to it, and yes, 4x electric weakness, but it's never been a big deal to ruin it competitively. Garchomp is 4 times weak to ice, does it ruin him? No, of course not.
Gyarados as water/flying has 2 weaknesses, 5 resistances and one immunity (immune or resisting the 2 best attacking types, ground and fighting).
Compare that to a pure water, 2 weaknesses still, but only 4 resistances, no immunities, not resisting the 2 best attacking types. Water/Dragon has one weakness, but a crappy 3 resistances and no immunities.
Apart from having the currently overpowered dragon stab attack, it's not better for Gyarados to have it, and if you think it is BECAUSE of that dragon stab attack, then all you're doing is saying how OP dragon currently is.
Flying type only 2 downsides is a stealth rock weakness and a limited attacking move pool, where the only real physical option is a recoil attack that most can't learn, while special attacks (hurricane aside) are also generally lacking, it is however, not a bad type to have.