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
-Hidden Power Ground
This might just look like a copy of its OU set, but it also works really well in UU. Double Edge is a nice standard attack, hitting just about anything not called Haunter hard. Hydro Pump is the obligatory water STAB, 2HKOing Nidoqueen. Thunder is your electric move, giving the tier's waters a nice "surprise" and is its best option against Slowbro. 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. Its move coverage hits everything the tier for at the very least neutral. Given its defenses, it'll probably be a while before it keels over (unless your opponent has Thunderbolt).
Set 2: RestTalk
-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. STAB attacks are used since it needs all the power it can get out of its slots. 95/79/100 defenses means it's not dying too soon.
The only other options here are Hidden Power Flying for STAB and Roar for phazing.
[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.