No disrespect, but in my (and many others') opinion, DT/O doesn't make a lot of sense on a Gyarados. If you want to fight Dragonite, you might as well use Ice, or once Gen3 comes out, Gardevoir might be very good at that too. DT/HP, Bite/HP, and Bite/Crunch (no STAB but fairly effective against Psychic and Ghost) all have their uses.
Further it is speculated that Gyarados might get Waterfall as a water based quick move, which would make Waterfall/Hydro Pump a really strong water attacker. But Gyarados may also lose Hydro Pump and get Aqua Tail in which case new Waterfall and legacy Hydropump would be the best moveset. Of course, they could also nerf Hydro Pump and buff Aqua Tail or Water Pulse, so nothing is certain. But having at least one Gyarados with Hydro Pump is good insurance against the move becoming legacy. If you can get a CTM and don't have somewhere else to use it, Gyarados to get HP might be a good option. But if you don't raid much, you might want to save a CTM for when you get Tyranitar with Fire Blast, Mewtwo with Hyperbeam, or something similar where there's one substandard charged move and you get it.
In that case, best advice is to try and find a Magikarp nest and move in there for the next few weeks, catching like crazy, using pinap berries on every 'karp, and hoping to get 400 candy and a good evolution candidate. Magikarp nests are usually near streams and rivers. There's a couple right near me, an old bridge over a stream, a park with pond, and a community college with a pond. My raid buddies report that the pier in Marcus Hook, and Penn's Landing, are both heavily populated with 'karp. So look to water for Magikarp, and also hope to get lucky and catch a few Dratini as well - Dratini are often found in the same places as Magikarp.
