Ice Fang.
Gyarados has not good special attack and coincidentally Blizzard is a special move type. Whilst Gyrados has a good physical attack, Teach it ICE FANG!
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Gyarados should learn Bite once Magikarp evolves at lv 20. Gyarados will then learn Dragon Rage at 25, Leer at 32, Hydro Pump at 41, and Hyper Beam at 52. Gyarados will also learn moves if you teach it TMs and HMs.
Gyarados cannot learn crunch by any means
Gyarados will learn Aqua Tail at Level 35.
Gyarados cannot learn water gun.
If you don't want it to learn any hm moves, then let it learn thrash, ice fang, bite, and use a TM a let it learn flamethrower.