I think you need to breed it. And that the gyarados you have now can't learn it. I think. When u do it pls send me a msg 2 see if it worked.
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Im not sure how or when gyarados learns thrash, but it does learn dragon rage at level 25 you have to get it at level 1..or move tutor with heart scale
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.
Gyarados cannot learn crunch by any means
Gyarados will learn Aqua Tail at Level 35.
Gyarados cannot learn water gun.