So, I recently released my personal VR for the meta, and the very first mon I mentioned there was Azumarill, and for good reason. I personally believe that with Exca, Conk, and PoryZ all being banned (not saying I disagree! I advocated for their bans for a reason), I do believe Azu is the unquestionable best non Ubers mon in the meta with there being a small gap between it and Gambit with the two big reasons it is ahead being versatility and typing.
As great as Gambit is, versatility is the one thing it lacks outside of its tera tera type. You will always see Swords Dance+Sucker Punch+2 of Iron Head, Kowtow Cleave, Low Kick, or Tera Blast. It will always be Leftovers, Boots, Lum Berry, Air Balloon, or Black Glasses. So as terrifying as Gambit always is, the one thing you can't really call it outside of terastalization is unpredictable. Now Gambit's tera type is where it is most unpredictable. Dark, fairy, flying, ghost, water, fighting, and fire are all common and have their own separate advantages, so let's see how Azu stacks up
Azumarill, on the other hand, has much more it can do a fair amount more. Aqua Jet, Liquidation, Play Rough, Tera Blast, Knock Off, Belly Drum, Ice Spinner, Trailblaze, Superpower, and Encore are all incredibly viable moves that see enough usage to where they all have to be taken into account. In terms of items, Azu also runs more. The big three are Choice Band, Citrus, and Assault Vest, but Azu can also run Covert Cloak, Ability Shield (specifically for the Weezings), Leftovers, Protective Pads, Lum Berry, or a resist berry to great effect. Azu's best tera types consist of water, fairy, ground, steel, electric to stop para, and fire to stop burn.
The second reason is typing, or more specifically, resistances. Kingambit's type is brilliant, there's no doubt about that, but Azumarill's typing and abilities match up far better against so many meta threats.
Kingambit's biggest advantages are against Dragapult (without Flamethrower or Wisp), Gholdengo (without Focus Blast), Hattrene, Iron Boulder (without Close Combat or Earthquake), Kyurem, Rillaboom, Roaring Moon (without Earthquake), Glowking, and Weavile (without Low Kick)
Azumarill's biggest advantages are against Barraskewda, Gouging Fire, Heatran, Kyurem, Meowscarada (without Thunder Punch), Ogerpon Wellspring, Rillaboom, Roaring Moon, Hisuirrot, Serperior, Volcarona, Walking Wake, Weavile (without Poison Jab), Blaziken (without Thunder Punch), Chien-Pao, and Urshifu Rapid (if it doesn't have Thunder Punch or Poison Jab).
As can be easily seen, Azu's good matchups are significantly more pokemon than Gambit's good matchups, and it is also less reliant on move choice from the opposing pokemon. This was just OU and Ubers pokemon as well, not taking into account mons like Hydrapple, Breloom, Hariyama, or Heracross.
One thing that I haven't mentioned yet is that Gambit won't have any bonus resistances when terastalizing. If you tera fairy, you have fairy's weaknesses and resistances, same with flying, same with ghost. Azu's ability combination, however, allows it to transfer a grass immunity and a fire and ice resistance on top of any tera type, giving it much more utility and leeway as a tera abuser.
While Gambit is definitely the number two mon in the meta (outside of Ubers mons), it just barely falls short of everything Azu has going for it.