Gyarados is a monster, yes, but it has a few tiny flaws that make it fall below A+ standard, which is why I would only support it up to A rank.
First off, it deserves a raise because of the reasons Smashbrosbrawl stated- its unique typing, stats, and ability give Gyarados all the tools it needs to be able to set up on nearly half the meta and sweep through pretty much the rest after a boost or two. Its SubDD set is really destructive, and is walled by a relatively view variety of Pokemon who must actually rely on resistances and not even just natural bulk to wall this monster.
However, that's where Gyarados falls short. It is an awesome and destructive sweeper, so it's definitely A-tier at least, but it does have a few small issues. Mainly, it requires significant support and has a few common hard counters as well as a weakness to Stealth Rock and low initial power / speed which makes it hard to set up and sweep early and mid game. Its biggest fall is in the fact that it really relies on Rain or at least the absence of other weather (which Rain can pretty much guarantee). Additionally, it needs the Rain boost to be able to muscle past some of its walls, such as Skarmory, before it can get crippled or forced out to be damaged by another round of Stealth Rock.
Speaking of walls, this is Gyarados's second major falling point. Sure, few Pokemon can reliably check Gyarados, but this number is not insignificant and many of those said walls and checks are actually pretty common. Skarmory, Forretress, and Ferrothorn, three of the most common Steel-types, all wall Gyarados to the sun and back, Taunt or no Taunt, Sub or no Sub, Rain or no Rain. (unless Gyara has boosts) Skarmory can easily sponge Rain boosted Wterfalls and force Gyarados out, while Ferrothorn can Power Whip Gyarados's subs and then cripple it with Leech Seed or Thunder Wave. Forretress can Volt Switch Gyarados for a lot of damage, and switch to a revenge killer to remove even a boosted Gyara. Other Water resistant Pokemon, especially SubToxicroak, can defeat Gyarados, as it is forced to use Bounce to hit them and that gives them a free turn to set up. Bulky water types such as Vaporeon can easily sponge attacks and force Gyarados out. Gyarados isn't exactly the type of Pokemon that can just switch in and start setting up again- as its SR weakness and need of a high HP amount means that sweeps are often dependent on one or two attempts and then Gyara has to be sacked. Unfortunately, Gyarados's counters are extremely common, so its sweeps will often be cut short, and its weakness to SR does not help it overcome that problem at all.
Finally, it has low initial stats. This might not seem significant as Gyara is a Pokemon built for boosting, but to put things in perspective, even a +2 SubDD Gyarados can't outspeed Scarfed Musketeers- and adding extra speed just means less power pr less bulk. Its low speed and relatively not outstanding Attack stat mean that it absolutely needs its Subs early in its sweeps in order to be anle to stop common revenge killers such as Keldeo, Terrakion, or Latios, three extremely common revenge killers that can defeat even +2 Gyarados as long as there is not Sub up, which is not a tough prerequisite to guarantee in the beginning or even middle of the match.
It's destructive and a good to check to a lot of things, and basically just has those few flaws, so I still support Gyarados to A or A-rank, but not A+.
