Ah, good old Gyarados. Such a reliable pokemon. So good.
Its tier placings throughout the years:
RBY: UU, only because thunderbolt was ubiquitous.
GSC: UU, only because at this stage Dragonite outcompetes it for what it can currently do. Also, it lost its 100 spatk, which was dropped to 60. Combos extremely well with Steelix.
RSE: OU, easily. This was still pre physical/special split, but Intimidate breathed a fair whack of bulk and threat into Gyarados. Its movepool also improved considerably at this time. Hated Zapdos, but most pokemon of this era struggled with Zapdos.
DPPt: OU, easily. Gained even more effective power with the physical/special split, as well as a fair amount of new moves to play with and items to benefit from.
BW: OU, easily. Practically unchanged. It's now got Moxie, so it can function as a wincondition against a weakened team even better than it did before, though it gives up effective physical bulk to do so.
XY: UU, with its mega being BL (or, banned from UU but not used in OU). A couple more counters emerged against it, and like in DPPt onwards, stealth rock ubiquity hurt it, but it was still a pretty damn top tier pokemon in UU, and often saw usage in OU anyway. Like Steelix, its partner in GSC, Gyarados appreciated Lightningrod ground types like Rhyperior or Marowak. XY was a period where prankster and priority ran riot, and Gyarados often struggled with priority attacks. Mercifully, there remains no priority electric quick attack variant. Gyarados was still a gargantuan threat in UU and at least a good threat in OU.
SuMo: bumped up to BL / OU again! Gyarados remained far too strong for UU. Its mega presumably got a bit more traction as more powerful poison types began to start to press the Fairy threat away, and its excellent bulk in mega form. It's bulky enough to take hits from high usage OU members like Protean Greninja or Mega Swampert. Like was mentioned by Umiman, the Mega form can even do quite well in Ubers, with Mold Breaker letting it break past stally teams, while also have pretty decent bulk and a nice typing to boot. It even checks Primal Groudon, with its 155 attack powered waterfalls!