Hello, I would like to offer my analysis on Garganacl and how I feel it should both be played around and evaluated as a member of the tier. This post will be quite long and will not feature a TL;DR, but I ask that you read it regardless.
Part 1: Garganacl's sets
Garganacl most commonly runs one of two boosting moves - Iron Defense or Curse - alongside its signature Salt Cure, Recover, and then one of Body Press or Earthquake. There are advantages to both of these. Iron Defense gives Garganacl a stronger offense matchup as it can keep up with boosting sweepers such as Swords Dance Garchomp and Ogerpon at the cost of a weaker matchup into bulky teams that feature Pokemon like Slowking, Toxapex, and Amoonguss which resist Body Press. Curse on the other hands requires a lot more setup and runs through more Recover PP while also weakening the matchup against offensive staples and Pokemon such as Hydrapple and Galarian Moltres, though it does allow Garganacl to more easily defeat Toxapex, Metagross, Iron Moth, and Pecharunt.
Garganacl's most common Tera types are Fairy and Ghost. Tera Fairy has a better matchup against Garchomp, Latios, Galarian Moltres, Greninja, etc while Tera Ghost has a better matchup against Pecharunt, Scizor, Ursaluna etc. - there are other much less common Tera types such as Water and Electric out there as well.
Part 2.1: Garganacl's counters
Garganacl is most commonly hard countered through the use of Covert Cloak on Pokemon that can handle Body Press or Earthquake. The best example is Sinistcha, with others such as Amoonguss, Latios, Pecharunt, Manaphy and Jirachi making appearances as well. Garganacl is also dealt with extremely well by Hydrapple and can be fended off with other Regenerator Pokemon such as Toxapex and Amoonguss which can force pressure onto it. Magic Guard users counter Garganacl, though the only legal one right now is Reuniclus. If you want a Pokemon that can switch into Garganacl every time and deal with it, these are the ones you'll want to use.
Part 2.2: Garganacl's checks
Checking Garganacl is a more nebulous ordeal and is quite difficult to do on paper, which is why Garganacl is such an overwhelming builder presence. However, here is a shortlist of threats to Garganacl that create issues for its user without hard countering it:
Entry hazards: Garganacl does not effectively run Heavy-Duty Boots, and UU's removal options are poor while its best Pokemon frequently runs double hazard sets. This makes entry hazards an enormous hurdle for Garganacl unless it commits to a pairing with Mandibuzz, which causes severe teambuilder strain and makes its matchup into Covert Cloak users even worse than usual.
Knock Off: Garganacl relies on its Leftovers more than most Pokemon do. A decent number of Knock Off users can either switch directly into Garganacl to force its item off, or trade Salt Cure damage for the hit. Some examples include Tornadus-T, Lokix, Okidogi, Metagross, Tinkaton, Mienshao, Cinccino, Scizor.
Strong (special) attackers: Garganacl is very bulky, but strong neutral hits on the special side are capable of forcing it out. Forcing Garganacl out with its item removed and entry hazards up creates a strong gamestate for the opponent; good examples of Pokemon that are able to do this include Latios, Keldeo, Manaphy, Iron Moth, Iron Crown. A number of very powerful physical attackers, such as Ursaluna, Kommo-o, Azumarill and Terrakion are also capable of doing this.
Encore: Forcing Garganacl to click a setup move or Recover can open free opportunities to switch into an Encore Pokemon like Tinkaton, Azumarill, Cinccino, Ogerpon, or Jirachi, effectively forcing Garganacl to switch.
Substitute: A few Substitute users such as Okidogi, Keldeo and Tyranitar can take advantage of Garganacl, especially with Tera. Most Pokemon can also "randomly" do this, allowing for effective luring.
Hitting it on the switch: While a bit more difficult to explain as this can vary so much, Garganacl needs to find opportunities somewhere and will usually find them on weaker special attackers like Tornadus-T or defensive Zapdos. By damaging Garganacl with these Pokemon on the switch - or using their pivot moves - you can force damage onto Garganacl and force it to heal, making it easy to get a Pokemon in without forcing Salt Cure damage onto it.
Part 3: Covert Cloak
I would like to address this item in particular as I see many players seemingly reducing it to "the Garganacl item". Covert Cloak is an item that is most often used for Garganacl. It sees much more sporadic use in metagames without Garganacl around. However, it is not useless - far from it. A large swathe of Pokemon in this tier use moves that have devastating secondary effects. Covert Cloak helps with an enormous amount of these, including but not limited to:
Flinches from Galarian Moltres, Greninja, Jirachi
Confusion from Tornadus-T, Pecharunt, Zapdos
Poison from Pecharunt, Amoonguss, Tornadus-T, Iron Moth I suppose
Scald from Manaphy, Suicune, Slowking
Paralysis from Zapdos
Luster Purge drops from Latios. Shadow Ball drops from Pecharunt, Sinistcha, Gengar
Every move Volcanion uses.
Ice Beam from Greninja and various other occasional users.
Speed drops from Tornadus/Keldeo using Bleakwind Storm or Icy Wind (notable for Latios).
In a vacuum these may not seem influenital. However, they make an enormous difference on the bulky Covert Cloak users in particular - Sinistcha, Pecharunt, Amoonguss for example. If you genuinely think that these are useless or just inconsequential byproducts, genuinely ask yourself; how many times have you been in an endgame where you need to dodge a flinch to win?
Covert Cloak is not some useless item, and Garganacl's existence does not force us to use something we "shouldn't". Not only can you beat Garganacl without it - Salt Cure is only 6% chip on Leftovers Pokemon, something Latios, Sinistcha etc. can very much handle - it also has strong applications in the metagame outside of Garganacl and improves consistency of most of its users.
On the topic of things we "shouldn't have to use" - we do not decide this. We are playing the game that we are given, and we do not simply decide what we should and should have to use on the basis of not liking it. I don't like fighting entry hazards and don't feel I should have to use Heavy-Duty Boots, a suboptimal item, to offset them - but I do, because that's the meta we've been given. Any arguments about whether or not Garganacl is broken should centre around if it's broken or not, not whether or not we should have to use an item to beat it. We also had actual users of Float Stone Gastrodon for Thundurus-T by the way...
Part 4: Garganacl's dominance in game
Here are all of the circuit playoff replay games in which Garganacl showed up. I will mark the Garganacl users in italic and bold the winner.
https://replay.pokemonshowdown.com/smogtours-gen9uu-738996 -
Mimilucha vs
Feliburn
https://replay.pokemonshowdown.com/smogtours-gen9uu-738997 - Mimilucha vs
Feliburn
https://replay.pokemonshowdown.com/smogtours-gen9uu-739280 -
hariyana grande vs jawabarat
https://replay.pokemonshowdown.com/smogtours-gen9uu-739380 -
Cam vs
starbitstorm
https://replay.pokemonshowdown.com/smogtours-gen9uu-739429 -
Parpar vs udongirl
https://replay.pokemonshowdown.com/smogtours-gen9uu-739591 -
robjr vs Dj Breloominati
https://replay.pokemonshowdown.com/smogtours-gen9uu-739593 -
robjr vs Dj Breloominati
https://replay.pokemonshowdown.com/smogtours-gen9uu-739864 -
Mimilucha vs Lily
https://replay.pokemonshowdown.com/smogtours-gen9uu-740649 -
JustFranco vs udongirl
https://replay.pokemonshowdown.com/smogtours-gen9uu-740655 -
JustFranco vs
udongirl
https://replay.pokemonshowdown.com/smogtours-gen9uu-740883 -
hariyana grande vs
robjr
https://replay.pokemonshowdown.com/smogtours-gen9ubers-741331 -
starbitstorm vs
justdrew
https://replay.pokemonshowdown.com/smogtours-gen9ubers-741335 -
starbitstorm vs
justdrew
https://replay.pokemonshowdown.com/smogtours-gen9uu-743013 -
JustFranco vs
Mimilucha
https://replay.pokemonshowdown.com/smogtours-gen9uu-743070 -
starbitstorm vs
hariyana grande
In total that's 15 games with a Garganacl in them and in total it's 8 wins for the Garganacl user with 1 ditto. Notably it was rare for Garganacl to run into a hard counter and it still found its way into losing positions at a regular rate. I do not wish to analyse every game as there is simply no point and others can draw their own conclusions, but notably Garganacl did not really take over any games here; it was never found on the winscreen.
Part 5: Miscellaneous finishing thoughts
I disagree with a number of arguments presented thus far about Hydrapple being Tera reliant or awkward to fit. These feel to me like they come from peolpe who haven't actually used the Pokemon and instead just focus on its negative traits and stick to them. Hydrapple rarely wants to use Tera and only does it commonly against balances that rely on Toxic to beat it - it can also panic button against Greninja, Latios etc. but it's rarely required. Hydrapple is excellent with its base typing and there are very few Fairy/Ice moves being thrown around to begin with, with Azumarill and Greninja being the only real common ones. Being weak to Draco Meteor sucks but it's not a death sentence and that's what you've got teammates for - not like the Dragons can switch into you either.
There are some other forms of counterplay I didn't mention like Chilly Reception into Encore or Future Sight + Chilly Reception with a strong special attacker. I didn't want to create a full point about them but they do exist and are an effective method of getting through Garganacl.
Garganacl being a Tera hog and only really fitting on balance structures in a metagame so heavily centered around offense results in low usage and so I think for anyone looking to actually significantly change the metagame this is the wrong angle to take. Suspect testing Latios would've made more sense in opening the tier up.
Garganacl is also one of the better anti-offense tools that helps with stuff like Galarian Moltres, Iron Moth, and SD Garchomp. Its presence is not *necessary* for these Pokemon, but fewer options to deal with them feels like taking the tier in the wrong direction.
I hope that this is in some way informative or helps others to make a decision. Personally I will be voting
Do Not Ban on Garganacl as I feel it is a healthy presence in this metagame that provides useful defensive value while not being overly constraining. I encourage others who feel the same way to speak up. Thanks for reading.