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.
The CP of the attacker is the number next to the names of Pokemon.