First of all it is hard do deny that Gholdengo's covert cloak set is mostly dedicated for Garg, like I said, Gholdengo makes the better use out of Covert Cloak because it also, matters against Clodsire, Breloom, etc.
I also want to highlight your contradiction, as first you say that the Gholdengo's spread and set is very common (as a way of claiming that this set is not dedicated) and later say that the spread is dubious and uncommon.
As you know, a set is a combination of item, ability (if variable), moveset and EVs. A set is not just an item, even it it can be its most important part, like with Choice items.
Of course Gholdengo wants to run Covert Cloak specifically for the Garg match-up, but if it wanted to specifically counter Garganacle "as a set", that is, "as its role in your team", it should run Thunderbolt instead of Shadow Ball to hit the most common forms of Garg (Fairy and Water super effectively, Steel and Ghost neutrally), specially considering that there's no scenario in which Shadow Ball would be useful against Garganacle. Also would want to run another Tera-Type; Flying is run to counter/better handle the match up against the Ground types I mentioned, NOT Garganacle. Flying against Garganacle is usually a bad idea, as it doesn't always run EQ, but it certainly always runs Salt Cure. You'd honestly consider a "dedicated set" to run a Tera-Type that makes you weaker to the one move you know the one mon you're trying to counter will run 100% of the time?
My point was that the set you showed was a mix of "all-purpose bulky NP" in moves alone, but with EVs that are better aimed at a more offensive role that doesn't want to run NP and Recover at the same time, like Trick Scarf (which ironically would be more effective if you manage to Trick Garg itself). A set that mixes traits of two different common sets is unusual, and often not very effective. The fact that the item considered Garganacle as a potential foe, but neither moves nor EVs were optimized for this match up means that the intention behind that set was not to counter Garg specifically, but rather considered Garg as just "one of many" things Gholdengo could face.
Also you say that Water Garganacl has to position itself perfectly against Covert Cloak Gholdengo and that the match-up is tricky, later showing damage calculations that prove your own conclusions to be wrong.
Only +6 Gholdengo is able to 2hko Garganacl with Shadow ball, and while you can still 2hko it with +4 Make it Rain, you are still too slow to pull that off. As you switch in and get to +4, 3 turns pass, meaning Garg was already able to get to +2 and hit you once, Killing Gholden the next turn. There is nothing tricky here, if you predict Garg to switch in and curse you are already in a position where you beat it 90% of the time unless ur opponent crits you or uses surprise Tera to get a free turn.
Honestly, I don't want to argue too much on this fight scenario because my biggest issue is with the Gholdengo set you presented itself; delving into this fight is assuming a premise I cannot accept.
But the situation you describe has already a Garg on the field (meaning a 1 turn advantage on the match up), predicting correctly an opponent switching to Gholdengo, then revealing your set (with Curse instead of EQ, to boot; if you predict Gholdengo anyway, EQ would just leave it in range of a 2HKO, forcing it to get locked into Recover, accept the loss or Tera and become exploitable with super effective Salt Cure; if it's Air Balloon, it loses to Salt Cure anyway. You have NO REASON to curse instead of EQ if you predict it), THEN having your opponent not understand that Curse on Garganacle makes EQ very likely and get completely blindsided by it.
Let's review that, turn by turn
Turn 1- Garg on field. Gholdengo switches in. Garg predicts and uses Curse. (Tera status unknown)
Turn 2- Gholdengo doesn't tera (for some reason) and uses Nasty Plot. Garganacle may Tera-Water in fear of Make it Rain or not, uses Curse again.
Turn 3- Gholdengo STILL doesn't Tera (whomever is controlling this Gholdengo really deserves to lose here, lemme tell you. What do they think Curse on Garg even means? and if you're facing a mon that gets OHKO by Make it Rain, and it stays on you, and it even dares to set up Curse instead of switching out, what exactly do you think is their plan? to die for laughs?), uses Nasty Plot again. Garganacle EQ and wins.
... now let's assume Gholdengo DID Tera at Turn 3
Turn 3- Gholdengo Tera-Flying. Uses Nasty Plot. Garganacle uses EQ. Does nothing.
Turn 4- Gholdengo uses make it Rain for ~65% damage. Garganacle uses Recover (you also deserve to lose if you use +2 Salt cure for only ~55% damage, forcing you to switch out into a +3 attack or lose Garganacle)
Turn 5- ??? Gholdengo can use NP predicting Recover, and Garganacle can use salt Cure predicting NP. Or they can both be straigthforward and use Make it Rain VS REcover again for little change.
This scenario was the one that becomes a pure prediction to see which wins, but Gholdengo has the advantage. This circumstance would change if Garg had tried to EQ before getting to +2, but again, Gholdengo could also Tera-Flying any time.
Now, let's assume Gholdengo Teras at the wrong timing:
Turn 2- Gholdengo Tera-Flying in fear of EQ and uses Nasty Plot again. Garganacle Tera-Water in fear of Make it Rain and Curses Agian.
Turn 3- Gholdengo uses Nasty Plot agian, as it's still not particularly scared. Garganacle uses +2 super effective Salt Cure for ~55% damage.
Turn 4- Gholdengo can use Make it Rain to heavily damage Garganacle and leave it ready to be revenge killed with easy, or use Recover in hopes that Garganacle does something weird, like Recover predicting an attack. Garganacle uses Salt cure again and either kills, but is left with very low HP, or deals ~55% damage again and the flow of this turn is repeated until either side gets tired.
Gholdengo loses the 1 on 1, but Garganacle is too weakened and would struggle to make itself useful for the rest of the battle.... but it's also relevant to note that these numbers on Salt Cure and EQ are only true for a Gholdengo set with no or almost no bulk investment; EVs alone can make this scenario a loss for Garganacle, as it would need another turn to get a kill although Gholdengo would be crippled with very little HP.
Note that this scenario is flipped by a Tera that isn't even the best for this situation, with a significant positional advantage on Garganacle of already being in the field, Gholdengo hard-switching into it AND predicting the switch.
So no, not simple at all.
You also mention the use of Covert Cloak on Corviknight and Skeledirge. I actually already tested Covert Cloak on these 2 Pokemon, and know very well they are not consistent. Corv is one of the best Knock Absorbers in the tier switching into Meowscarada, Great Tusk and Iron Treads and losing its item on the process, not being able to wall Garganacl anymore (not to mention it loses to iron defense/curse sets regardless). Skeledirge also hates losing HDB, and with team synergy and Hazards up the Garganacl player will just predict Skeledirge to come inde eventually and start switching to a Check to hazard pressure it. In fact, with hazards up, you dont even need to predict it, just salt cure and switch out it eventually runs out of recovery.
That logic is wrong at its base. First, Covert Cloak Corviknight must value its item highly if the opponent is using Garganacle as it needs it to counter it. You're simply missplaying if you send Corviknight into a potential Knock Off when you know you want to keep your item. He may be a Knock Abosrber usually, but you can't have him be a Knock absorber and a Garganacle counter at the same time, and if you prioritize wanting to send it on Knock Offs, you shouldn't equip it with a Covert Cloak to begin with. But that's just part of the game; sets countering each other, understanding the position and the situation of the game to know what parts of your team are more valuable in a given match up and what parts of your opponent's team are more threatening and adapt accordingly. Also, "not to mention it loses to ID/Curse sets regardless"; is this line meant to refer that Garganacle wins against Corv even with Covert Cloak? Because that's blatantly wrong. Unless it's a pure pivot defog support Corviknight, which I'd argue is it's worst set by far in a meta with Gholdengo hard-countering it and threatening to use it as setup fodder. If Corviknight is running Iron Defense/Body Press itself, then the side with full Def investment wins, and if both/neither have Def investment, Corv wins because neither side has enough damage to break past the other, and Garg runs out of PP first due to Pressure. And if Corv isn't running ID/BP, then why give it a Covert Cloak to begin with? What does Brave Bird/U-Turn/Defog/Roost Corv do in front of Garg other than look stupid? Why would you even consider using the item slot on a match up you have no answer to?
The Skeledirge argument is, I'm afraid, straight up outrageous. The "only" conditions you set for Skeledirge to stop being reliable is an entire team effort to force it in and out, prior Knocking it off, with hazards on the opponents field (and only on their field, given how quick you are to switch around) and constantly predicting when it comes out? Well, thank Arceus it
only takes that!
A more constructive argument would be a discussion on whether Substitute would be a good option for it instead of Will-o-Wisp/Yawn in the current meta and if this set becoming popular would be testament to Garganacles presence. Or if it would be worth it to run Covert cloak on Skeledirge, assuming you'll probably have to Tera into Water/Fairy anyway and don't care (as much) about hazards after that. Ignoring that this Skeledirge would also have other pokemons behind it and a player behind that can predict you too is just not a serious argument.
garganacl with 
heatran
shed shell
gholdengo for 
toxapex
Slowking + a substitute set up user is basically feasting on garganacl. Whether you decide to use 
orthworm to set the sub in order to save valuable moveslot spaces, or just straight into a viable sub + setup user.
hatterene with a Tera up her sleeve. She tends to break through 3+ Pokémon on a typical gargancl team, especially if it’s +2 out the gate.
