Well, since MF told me to answer this instead of just commenting..
Actually, there is reasoning to it. Gyarados was modeled after Chinese Dragons(which can fly), but if it were a Dragon Type, well....when Gyarados was made, there was only one Dragon Type move, Dragon Rage, which always does 40 Damage. Since Gyarados's Water Type would have made Ice Type moves nuetral against it, Gyarados would have been nearly invincible. That is why it was made Water/Flying Type.
Source: This has been asked a million times, and many answers(including ones with Bulbapedia sources) said so.