Gyarados really seems like a Pokemon that would be a really good Pokemon in OU. It's got amazing stats (sans that awful 60 special attack)-being tied with Snorlax, Kingdra, and Blissey for the 4th highest base stat total outside of Ubers, a very nice typing (watch out for Thunderbolt though), and a decent enough movepool. It can also switch in on earthquake (which is everywhere) and takes no damage from spikes. This Pokemon strikes fear into a good chunk of the meta just because of how amazing its coverage is. So why is it UU? It's simple, Dragonite outclasses it in almost every way. Still, with Dragonite banished to BL, Gyarados can do whatever it desires with nearly nothing stopping it. This acts great as a late-game cleaner, allowing it to kill off weakened Pokemon with its great speed tier (only being outsped by 6 of the 20 top Pokemon in UU, not including itself), and when those Pokemon which do outspeed it are very frail, making its role even more defined as a cleaner.
Obviously Gyarados is not perfect. Its horrid physical movepool (which has a grand total of 3 good moves) makes it rely on its pathetic special attack, which obviously won't be killing anything fresh off the bat (even with a crit). In addition, it's killed by just about everything with Thunder, owing to its 4x weakness to it. Still, if you can look past that (as well as keep it away from anything you even suspect has Thunderbolt), this little thing will be a big asset to your team, and you won't regret that decision.
Set 1: Mixed Sweeper
Item: Leftovers/Miracle Berry
-Hidden Power Ground
This might just look like a copy of its OU set, but it also works really well in UU. Return is an all-around nice damaging move with no downsides, although Double-Edge can be used if you want slightly more power (watch out for recoil though). Only use Double Edge with Leftovers, as using it with Miracle Berry definitely isn't the best idea. Hydro Pump is for STAB, it gives most rock and ground types a run for their money. Zap Cannon is your electric move, it scares other waters and flying away, although that 50% accuracy makes it very shaky, Thunder can be used if you hate missing (and also has a bit more extra power), but it misses out on the paralyze chance that Zap Cannon offers. Thunderbolt can be used if you really hate missing, although its low power makes it undesirable. Hidden Power Ground is used to screw over Electrics such as Ampharos and Electabuzz.
The aim of this set is to just try and kill everything it can. Given its defenses, it'll probably be a while before it keels over (unless your opponent has Thunderbolt). Miracle Berry also an option as it really hates paralysis.
Set 2: RestTalk
Item: Leftovers (obviously)
-Hidden Power Ground/Hidden Power Flying
This set just basically tries to maximize this thing's longevity, and can be more of a mid-game bulky attacker as opposed to a late-game cleaner. Use Double Edge on anything that doesn't resist it or is called Haunter, use HP ground on anything that does. Hidden Power Flying is an option for STAB as well as it dealing more damage to bug, grass, and fighting types, however Normal+Ground coverage is unresisted in UU. It's up to you though. Use Rest at low health, use sleep talk, rinse, then repeat. 95/79/100 defenses mean it's not dying too soon (unless hit by a STAB Thunderbolt). Gyarados isn't really missing those other 2 slots anyway.
It can run a phaze set with roar to screw over stat boosters like Scyther or Victreebel, Curse is there if you can Electrics out of the way, HP Flying is your Flying STAB, and it's there for grass and bug types, Toxic is there if you want to stall, Endure+Flail (or Reversal) could be a neat option if it gets an Agility baton passed to it from Scyther.
[Checks and Counters]
As said many many times before, most anything with an electric move (mostly Thunderbolt or Thunder, though you might see Thunder Punch on Magmar or Kadabra) will at least 2HKO it, especially stuff faster than it. Special mentions go to Electabuzz, as it is both faster than Gyarados and OHKO's it with Thunderbolt, and Ampharos, which Gyarados can't do much to (HP ground is only a 3HKO on Amphy), while Amphy OHKO's it in return. Gyarados also really hates being paralyzed, as this makes it more vulnerable to getting hit by an electric move, be it Thunderbolt or Thunder Punch. The RestTalk can't even do anything about it as once it's paralyzed, it's basically a free switch in for an opposing Electabuzz to kill it.