The other answer is outdated due to the DLC (2.0 update) that came out later. Gyarados can be found in the new Mightywide River (Day) at levels 2 or higher. The way you find it is by hitting a hidden Crystabloom behind the center of a waterfall with an Illumina Orb, and he should appear. However, it only exposes some parts of its body, so you can't really take a very good shot there, but Gyarados will pop out of the water near the ending of the course, where it will most likely reveal itself to the right of the ending zone, where you can take way better pictures of it.
Note: While the Crystabloom is there in level 1 of the course, Gyarados will only appear in levels 2+.
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