*rubs temples* ... and not a thread goes by without somebody saying that Gyarados should be Water/Dragon. Just as well I like him and like typing this up all the time...
Magikarp and Gyarados are based on an ages old Japanese tale. The tale features a weak and defenceless carp... yes, just like Magikarp... prove that he has some strangth and will in him by scaling a waterfall. This was watched by the Gods, who rewarded this carp by turning him into a flying serpent... just like Gyarados.
Now just a few more pointers. I'm well aware that Gyarados cannot learn flying moves. However the serpent that he was based on could very easily. Plus the serpent in the myth abused his new found power by destroying nearby cities... hence why Gyarados's movepool is packed more with destructive, high powered moves like Fire Blast and Hyper Beam.
So you may not know of mythology and will probably never read it. However I know and do, the tale exists and that tale was what inspired Magikarp and more importantly, Gyarados. He isn't going to change his type... and for the record, in Japan and China their serpents do look like Gyarados. Their dragons always have legs.