Gyara is a pretty cool dude in RBY OU. Not standard by any means, mostly because it's a physical attacker without STAB Normal moves. Still, it can catch Rocks on predicted Earthquakes and then go to town with its fantastic mixed attacking stats, great Special attacking movepool including STAB Hydro Pump, Blizzard, and Thunderbolt, and of course a Physical movepool including Body Slam and Hyper Beam for killing Chansey. That STAB Hydro Pump is especially nice for letting it beat down on Normal-type attackers in one-on-one matchups. Overall, Gyara isn't fantastic, but it is a nice, viable little attacker that pretty much nothing can switch into safely (except maybe Gengar).
In OU? Garbage. Not much I can really say about it, nobody uses it because pretty much everybody realizes how trash it is. As a mixed attacker, it fails because it has abysmal Special Attack and a shitty Physical movepool, and as a defensive role-player, Suicune does anything it can do, but much better, and then some (except for walling Heracross, I'll give Gyara that). In UU, I've heard from shrapn3l that it's pretty good, but Ive never really played GSC UU so I can't really comment on it. My guess is it probably functioned as a sturdy Phazer for stuff like Growthtoed, and it likely utilized Curse. Or not, this is UU after all, it might very well have just been a pretty bulky mixed sweeper that could also switch into and check a lot of prominent threats.
Hooray, Gyarados is now good. Dragon Dance, Taunt, and Indimidate did wonders for Gyarados, and the EV system coupled with Celebi's drop to OU allowed for Gyarados' brand of Flying STAB offense to make a significant impact. It makes a great Heracross check and can set up on a lot of things that its more popular cousin, Salamence, can't; opposing bulky waters can't touch this thing as it sets up. Still, lack of Rock Slide and the lack of any really useable Special moves (except maybe Hydro Pump?) really put a damper on its coverage movepool, making it quite a bit easier to wall than other Dragon Dancers, especially with Zapdos and Skarm being pretty popular dudes. In metagames where Bulky Waters are rampant and Zapdos isn't all the rage, though, Gyara shines.
Welp. Gyara can now actually use its Water STAB again... and this time, it's off its better attacking stat. Having STAB Waterfall is a huge boon to Gyarados offensively, as is the new movepool addition in Stone Edge (and I guess Bounce, too, for restoring what Hidden Power Flying did). Plus, the overall increase in the effectiveness of attacking types that it resisted made it a much better defensive mon than it ever had been before. Its biggest problem here is the introduction of Stealth Rock, but paired with Rapid Spin support, it still shines in whichever role (offensive or defensive) it's put into. Definitely a top OU here, a status it's probably never enjoyed except maybe in early RSE (and possibly some in-game tier lists).