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English Answer:
Yes, Gyarados can learn the move Flail. Flail is a physical move that deals more damage the less HP the user has. It's an effective way for Gyarados to capitalize on its remaining health in battle. In the games, Gyarados can learn Flail through various methods, including leveling up, Technical Machines (TMs), or breeding with a compatible Pokémon that knows the move.

Chinese Translation:
是的，暴鲤龙（Gyarados）可以学习到招式挣扎（Flail）。挣扎是一个物理招式，使用者的HP越少，造成的伤害就越大。这是暴鲤龙在战斗中利用其剩余HP的有效方式。在游戏中，暴鲤龙可以通过多种方式学习挣扎，包括升级、使用技能机器（Technical Machines, TMs）或者与知道该招式的兼容宝可梦繁殖。

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That's not true that if you didn't evolve Magikarp until level 30 you'd be missing out other moves which Gyarados would learn, because you can always teach them with the Move Reminder. No. Flail is not worth it. Gyarados learns much more powerful moves between levels 20 and 30, totally invalidating Flail.read more >>