0130 Gyarados

Dragon Breath is a blessing to virtually anything that has it, and Gyarados is definitely no exception. It sports 4 DPT (Damage Per Turn) and 3 EPT (Energy Per Turn), which makes it Gyarados' objectively best quick move despite lacking stab. 

Aqua Tail (50 Power/35 Energy) is an incredibly cheap move that can provide frequent shield pressure. Hydro Pump (130 Power/75 Energy) can threaten KOs when not shielded, but its high energy cost makes it a less consistent option. Crunch (70 Power/45 Energy) provides good overall coverage, decent damage, and a reasonable energy cost, while Outrage (110 Power/60 Energy) is very powerful at a fair cost. This set provides Gyarados with a powerful catch-all playstyle that's hard to come into, and is likely to deal meaningful damage even to Gyarados' counters, while also allowing it to act on opposing dragons while not being hindered by a dragon weakness itself if packing Outrage. Return can be used as a coverage move against Primarina.

You could also go for Bite, with 4 DPT and 2 EPT, making Gyarados an aggressive Dark attacker. It is worse against Giratina, however, and with it Gyarados becomes even slower. Waterfall is also not a bad move, and is better against Steel and Fairy Pokemon, but there are generally many superior Water-type attackers to choose from.

Gyarados is a great Pokemon, but the Great League's CP restrictions really limit it. It's possible to leverage its typing and good moveset against select counters, but it's probably better to level it up to a higher CP and use it in another league.

Gyarados starts to shine in the Ultra League as the restrictions to its high CP begin to loosen. Here, Gyarados's single strongest niche has to be its ability to take down Giratina and wall Registeel. It can also remove the threats from Dragonite, Swampert, and Charizard. Gyarados' has access to great moves of Water, Dark, and Dragon type, all of which have very good coverage that is narrowly resisted when brought together.

In the open Master League, Gyarados functions as a generalist due to its versatile moveset and its Water/Flying typing. However, it struggles to actually stand out compared to the plethora of other Dragon-type Legendary options as Gyarados lacks their sheer strength. Gyarados has a tough time standing out when Lugia, Dialga, and Palkia exist. Its only real niche comes from its accessibility, and it can be useful for newer players to pick up due to its decent matchup spread.Gyarados is more potent in formats with restricted rulesets, as its general coverage can be valuable when it has less competition.

Such example of a format is Premier Cup, where Legendaries and Mythicals are banned. Gyarados's versatile coverage, strong typing, and good stats allow it to rise to the top of the metagame.