Gyarados is a Water/Flying dual type Pokemon.
It evolves from Magikarp starting at level 20. It evolves into M Gyarados at night, if holding a(n) Mist Stone.
The combination of Dragon Dance, Waterfall, and Dragon Ascent gives Gyarados an incredible setup move couple with two excellent STABs. Moxie offers even further passive setup and can snowball easily. Gyarados's wide movepool allows for a huge range of coverage options, including Ice Fang, Power Whip, Earthquake, Iron Head, and Outrage.
Mitigating Gyarados' 4x weakness to Electric can be done once with a Wacan Berry, but a strong physical attack may be able to OHKO from full health. Focus Sash can allow for a guaranteed use of Dragon Dance, but even at +1 speed it can be outpaced by fast scarf users.
Moxie is the go-to ability since it offers passive setup opportunity in conjunction with Dragon Dance to make Gyarados a more efficient sweeper.
Gyarados' useable Special Attack stat and access to Pride can make it a niche special sweeper. Life Orb is needed to give extra power to moves before Pride kicks in, although Choice Specs or Scarf could be used as well in late-game and revenge-killing scenarios respectively. A Timid nature helps Gyarados eke out a little bit more Speed, while Modest gives it the most power behind special moves.
For an even greater power boost, Gyarados can be used on rain teams to ensure hits on Thunder and Hurricane, as well as having a rain-boosted STAB Surf. For slightly less power, Scald can be used as well for a burn chance to help cover its mediocre defense stat. Ice Beam and Fire Blast are both moves with strong coverage, and other less powerful options for coverage include Dark Pulse, Steam Ball, Dragon Pulse, or Primal Noise. Icy Wind could also be used for speed control, but it is not advisable due to the low base power.
For both sets, Electric and Chaos move users give Gyarados a hard time, especially when preying on Gyarados' weaker physical attack stat. Additionally, bulky Water and Ice-types can force a switch due to the lack of reliable coverage against them.