this dedicated set loses to a very consistent Garganacl set, which does not have to face Covert Cloak Gholdengo to be good
I don't think there's a single part of this line that is correct.
1: "Dedicated Set"
The only thing of that Gholdengo that is "dedicated" to check Garganacl is Covert Cloak. Neither the EVs nor the moveset have anything to do with Garg, being actually a pretty standard Gholdengo bulky NP. And even that description is dubious; you'd probably be better off spending EVs on bulk instead of SpA if you're going to depend on Np+Recover. Flying Tera on Gholdengo is the most dubious part; that's something run exclusively to give a harder time to Iron Treads, Great Tusk, Clodsire and partially Garchomp. And Gholdengo doesn't like to stay on the first 3 anyway because of Knock Off, Volt Switch or Rock coverage(Treads), Ice Spinner (Tusk) and simply being hard-walled by Clod and countered if it's running ANY attack in addition to/instead of EQ.
2- "consistent Garganacl set that doesn't have to face Covert Cloak Gholdengo to be good"
Aside from getting OHKO by CB Meowscarada at any level of set-up
252 Atk Choice Band Meowscarada Flower Trick vs. 252 HP / 4 Def Garganacl on a critical hit: 366-432 (90.5 - 106.9%) -- 43.8% chance to OHKO
(94% OHKO if Adamant, not to mention hazards)
you also get 2HKO by CS Iron Valiant with Thunderbolt, who doesn't really care about Salt Cure since their bulk is mostly decorative.
252 SpA Choice Specs Iron Valiant Thunderbolt vs. 252 HP / 252+ SpD Tera Water Garganacl: 214-254 (52.9 - 62.8%) -- 99.6% chance to 2HKO after Leftovers recovery
Another good option for this is Rotom-Wash. Can't switch into Salt Cure without taking heavy damage, but can Volt-Switch right away and chip it if defensive, can kill if Specs.
Other stuff include Iron Moth or Volcarona, with Specs/Quiver Dance with Tera Grass, which is more situational but Tera Grass has always been a good option on Fire mons.
As for defensive mons, most things with Cloak works against it, too. Skeledirge works naturally against ID/BP sets because Salt Cure is only 40 BP and if it happens to be this one Curse/EQ set, you're not gonna care about the boosts, can tera out of EQ weakness and Torch Song is gonna make you snowball faster and better, even resisted.
0 Atk Tera Water Garganacl Earthquake vs. 248 HP / 252+ Def Unaware Tera Fairy Skeledirge: 52-62 (12.6 - 15%) -- possible 7HKO
(note that, even without Tera, Skeledirge takes around 30% HP from EQ, which is enough to have Skel's Slack-Off and Garg's EQ run out of PP, but still doesn't ensure a kill)
Amoongus (again with Cloak) also works wonders as it can take EQ with little trouble
+3 0 Atk Tera Water Garganacl Earthquake vs. 252 HP / 172+ Def Amoonguss: 176-208 (40.7 - 48.1%) -- guaranteed 3HKO
(made the calc on +3 to specify "when" does Amoongus have to start being careful)
Recover while attacking with Giga Drain, and even clear out Curse with Clear Smog.
but best is, of course, ID Corviknight, the second best user of Covert Cloak and the best Garg check. Cloak Corv will always run Pressure, which means you're gonna get PP Stalled out of Salt Cure if you dare to stay, EQ obviously doesn't work and Garg is gonna be set-up fodder for ID. Can even run Bulk Up+Power Trip Corv since you win the match up anyway and now your opponent's Garg looks stupid while you build up a win-con that is not stopped by Gholdengo any more.
Hell, you can even go for something weird like Iron Defense/Body Press/Power Trip/Roost to maximize on Corv's bulk ASAP and threaten Gholdengo/Dragapult/[insert random Ghost-Tera] that will not expect Power Trip on Iron Defense set (and they shouldn't, you have to be +6 for this to be mildly threatening and still fails to reliably 2HKO Gholdengo, but point is, you could).
Like, the only way you're gonna make such a time-consuming set that has Salt Cure as it's only way to make any progress (albeit, it is a very good move to do so) is by making it face Gholdengo. Which kinda tells me, going back to the previous point, that this Garg is the one dedicated to counter Gholdengo, and not the other way around. ID/BP garg has more immediate firepower and works well with Purifying Salt to not be exploited by Ghosts, and SR+Protect is just much more consistent, giving up entirely on doing anything to Covert Cloak mons, scouting what mon in your opponent's team has it to try to Knock it Off, and exploiting Salt cure's effect to its fullest with Protect against any other mon. This set is not even a fraction as consistent as that.
The fact that we don't have many Special Attackes in the tier that are good against Water types is the reason why this mon on Tera-Water (and dondozo, while we're at it) feel hard to crack, but it's not like we don't have any options. In fact, I'd say Tera Elec and Tera Grass for offensive Special Mons should be considered a priority in the current meta, and not just because of Garg as it also works on Dozo, Corv (Elec)/Tusk (Grass), and any Tera Water (the lost Anniliape, now Garg, maybe Skeledirge...)
3, "LOSES TO"
My favorite problem is that that's not even accurate:
+6 252+ SpA Gholdengo Shadow Ball vs. 252 HP / 252+ SpD Purifying Salt Tera Water Garganacl: 226-267 (55.9 - 66%) -- guaranteed 2HKO after Leftovers recovery
+4 252+ SpA Gholdengo Make It Rain vs. 252 HP / 252+ SpD Tera Water Garganacl: 253-298 (62.6 - 73.7%) -- guaranteed 2HKO after Leftovers recovery
+3 252+ SpA Gholdengo Make It Rain vs. 252 HP / 252+ SpD Tera Water Garganacl: 211-249 (52.2 - 61.6%) -- 97.7% chance to 2HKO after Leftovers recovery
0 Atk Tera Water Garganacl Earthquake vs. 20 HP / 0 Def Gholdengo: 150-178 (46.8 - 55.6%) -- 74.6% chance to 2HKO
+1 0 Atk Tera Water Garganacl Salt Cure vs. 20 HP / 0 Def Tera Flying Gholdengo: 134-162 (41.8 - 50.6%) -- 1.2% chance to 2HKO
this is a VERY complex fight, where timing matters for everything. Garganacl cannot allow Gholdengo to get 2 NPs and is lost if they get 3. Gholdengo cannot get hit by EQ twice or after boosting. Gholdengo can Tera into flying on a predicted EQ and get a free turn, which is likely to happen if Garg tries to Curse cuz you'd be revealing your set, and no way you're using Curse with only Salt cure/Body Press. And Garg needs to have found a chance to use Curse once to force Gholdengo to run out of Recover, and twice to actually threaten to kill.
Who enters the battle when? Have you tried using Salt cure first to scout Cloak? when do either of these mons Tera? When do you predict each move? ALL of these questions become relevant in this 1v1. Not gonna explain all the ramifications of this fight because this is already a big enough wall of text, but it's not a simple win for Garg.
Which takes me to point 2: that Garg is most likely fine-tuned to counter Gholdengo with Cloak, where as this Gholdengo is running suboptimal Ev, Moves and specially Tera-types to counter Garg. And EVEN THEN it's a close match. Sure, that set MAY win against Gholdengo, but mess up the timing or a read and you might get a trade at best, and a loss to a set-up Gholdengo at worst.
Now, I do agree that Garg deserves a Suspect test, but I'd lean towards no ban. Even if we do, I don't think it'd stay banned if we get more special attackers with offensive options against Water mons, which we desperately need.
And as for Covert Cloak, I do can see the argument about Garg being unhealthy to the meta if we're forced to run one mon with it in each team, but as someone else pointed out in the past, we wouldn't be discussing how Cloak is a bad item tailor-made to counter Garg if we still had the meta filled with mons with Scald. Same for the much more recent statement on how it helps against Serene Grace abusers, although I consider Scald a more relevant example.