Gyarados can use dragon-type moves despite its a water- and flying-type Pokemon. Those moves are super effective against Dragonite. Therefore, Gyaraods has an advantage.
Now Gyarados may not be the best Pokemon to beat Dragonite since it has only Bite for fast move. The most recent update take Dragon Breath away Dragon Breath away from Gyarados. For the details of the Pokemon Go update, tap the link below.
Gyarados is considered as the best Pokemon to defeat Dragonite, one of the most powerful Pokemon based on higher CP, due to its dragon-type moves. Here is the video of a Gyarados vs a Dragonite and the description of it.
Attacker
Note:
The CP of the attacker is the number next to the names of Pokemon.
Despite it is a flying- and water-type Pokemon, Gyarados can use Dragon Breath, a dragon-type fast move. The move is super effective against dragon-type Pokemons.
On the other hand, Dragonite use Dragon Breath or Steel Wing. Dragon Breath damages Gyarados as usual, but Steel Wing cannot damage water-type Pokemon well and the power is cut off by half.
As well as its fast move, Gyarados can use a dragon-type charge move, Dragon Pulse, and the move is super effective against Dragonite.
Dragonite use Dragon Pulse, and the move damages Gyarados as usual.
Because of Dragon Breath's higher EPS, Gyarados can use charge move relatively easily.
In summary, when Gyarados acquire Dragon Breath and Dragon Pulse, it is the best attacker to take over a gym defended by Dragonite, even if Dragonite's CP is higher than Gyarados.
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How to beat
Tap the name of each Pokemon to see the best Pokemon to beat it.