A: A lot of these pokemon can either not switch in effectively or not actually deal with gliscor. Of these, I counted 7 that couldn't deal much damage to it, and only 5 that can switch in WITHOUT being (very, none of them like Knock anyway) threatened (Corv Amoon Hatt Clef and Take Heart Manaphy). Amoongus and Corv are also on the list of "can't deal much damage to it", literally getting pp stalled with Giga Drain. This makes it an uphill battle to even get in against it because often, a toxic'd pokemon or a pokemon with its boots knocked is as good as dead in this chip-heavy meta.
Dozo you also forgot. Ohh and ghold and heatran. You are asking for switch ins, but for what set? I already explained that toxic, eq, and knock are not all run. 2 of the 3 are. And you can scout to see what the moves are. Different sets have different switch ins. If its EQ and toxic, you dont have to worry about knock. If its knock and toxic you don't have to worry about EQ. If its EQ and knock you don't have to worry about toxic. Iron valiant works as both a physical, mixed, and special attacker with many different sets. There is maybe 1 switch in for most sets (not all though) which is pex. Different sets require different answers. You are looking for a poke that deals with all 3 moves when it only ever runs 2. Also you mentioned amoonguss can't deal that much damage. That is true, but with synthesis you can just PP stall it out of EQs. And you can also use it to scout.
B: Even if you successfully predict or pivot into a sweeper, it's still laid those spikes that are likely not going away even if you have hazard removal like Tusk or Corv because of the funny string cheese man, which makes it harder for those pokemon to switch in again and threaten it. That's the problem with Hatt, why it's not being seen as high as its counterpart Clef: it takes that spike + eq damage and it's only recovery is lefties and draining kiss, both of which are really weak. For a gliscor answer to be a gliscor answer, it has to have longevity to match gliscor's, which, shockingly enough, trying to outrecover a pheal mon with 8 million sources of chip damage is kinda hard.
Or it could just okho gliscor. That also works...
you do realize that we were MUCH better when it came to dealing with hazard control pre-dlc because tusk wasn't forced into running ice spinner right? cus ya know knock off provided a good midgroud that hit the balloon ghold hard enough to the point that it would actually fucking die to eq after it got knocked? unlike ice spinner which doesn't do enough to ghold to actually put it into ghold range for defensive tusk? and the whole point of running spin tusk is to have role compression by getting your rocker AND spinner that happens to help check king gambit at the same time.
You can still run knock off on an ice spinner set. Knock, rapid spin, ice spinner, and headlong rush. That can deal with ghold while running ice spinner.
Yeah buddy nice one.... using a manaphy SURF calc to prove a point that gliscor cant stay in on the manaphy.
Surf is used on manaphy 37.581% of the time. Its not that uncommon. Scald is 61%.
As I have stated a few times now lets only 2 of knock off, toxic, and EQ. Now I will go indepth with calcs for most of my pokes listed before.
Corviknight (can switch in on all) Can clear hazards. Has some trouble pressuring gliscor. Roost gives it nice longevity
Tornadus Therian (can switch in) use gliscor to set up with NP and then threaten gliscor. Has longevity with regenerator. Doesn't mind toxic too much. Knock + toxic is probably the best glicor set against this but then you have other answers.
+2 252 SpA Tornadus-Therian Bleakwind Storm vs. 244 HP / 12 SpD Gliscor: 363-427 (103.1 - 121.3%) -- guaranteed OHKO
Dondozo (doesn't like getting knocked but honestly is fine if it does) (run avalanche to deal damage) (can switch in) Rest heals toxic.
0 Atk Dondozo Avalanche vs. 244 HP / 252+ Def Gliscor: 216-256 (61.3 - 72.7%) -- guaranteed 2HKO after Poison Heal (if hit with an attack)
0 Atk Dondozo Avalanche vs. 244 HP / 252+ Def Gliscor: 112-132 (31.8 - 37.5%) -- 52.8% chance to 4HKO after Poison Heal (not hit by an attack)
Hatterne (can switch in) (prevent hazards) (I still think its a decent counter. I don't want to argue it anymore but wanted to include it here for completion sakes.) (recovery with leftovers and draining kiss)
0 Atk Gliscor Earthquake vs. 252 HP / 204+ Def Hatterene: 81-96 (25.4 - 30.1%) -- 0.5% chance to 4HKO after Leftovers recovery
0 SpA Hatterene Draining Kiss vs. 244 HP / 12 SpD Gliscor: 88-105 (25 - 29.8%) -- possible 6HKO after Poison Heal
MG
Clefable (can switch in) (we all agree here)
Amoonguss (can switch in) (trouble doing damage, but honestly can just PP stall gliscor)
Manaphy - 252 SpA Manaphy Surf vs. 244 HP / 12 SpD Gliscor: 306-362 (86.9 - 102.8%) -- 18.8% chance to OHKO. Take heart sets also can run sleep talk and rest.
Greninja - 252 SpA Life Orb Greninja Surf vs. 244 HP / 12 SpD Gliscor: 408-484 (115.9 - 137.5%) -- guaranteed OHKO
Walking Wake - 244 SpA Choice Specs Walking Wake Hydro Steam vs. 244 HP / 12 SpD Gliscor: 474-558 (134.6 - 158.5%) -- guaranteed OHKO
Weavile (Ohkos gliscor and outspeeds)
Ninetales-Alola (blizzard/freeze dry goes burrr) 0 SpA Ninetales-Alola Freeze-Dry vs. 244 HP / 12 SpD Gliscor: 316-376 (89.7 - 106.8%) -- 43.8% chance to OHKO
Slowking galar One hit kos, has recovery with regenerator and does not take toxic and survives EQ
0 SpA Slowking-Galar Ice Beam vs. 244 HP / 12 SpD Gliscor: 352-416 (100 - 118.1%) -- guaranteed OHKO
0 Atk Gliscor Earthquake vs. 252 HP / 252+ Def Slowking-Galar: 170-204 (43.1 - 51.7%) -- 7.4% chance to 2HKO
If they are not running knock off:
Air Balloon
Gholdengo Untouchable and can set up and KO with MIR, shadowball, etc
Air Balloon
Heatran Untouchable and can trap and ko with magma storm
If they are not running EQ
Kingambit can set up on gliscor. Honestly doesnt mind lefties being knock too much
+2 252+ Atk Kingambit Kowtow Cleave vs. 244 HP / 252+ Def Gliscor: 195-229 (55.3 - 65%) -- 96.9% chance to 2HKO after Poison Heal (0 allies fainted)
0 Atk Gliscor Knock Off (97.5 BP) vs. 0 HP / 4 Def Kingambit: 28-34 (8.2 - 9.9%) -- ( with the item)
Toxapex has trouble dealing damage but cant be touch much at without EQ from gliscor
Iron Treads (can switch in)
Snealser can set up infront of it
Gholdengo can set up infront of it. Recover, NP, KO
0 Atk Gliscor Knock Off (97.5 BP) vs. 0 HP / 0 Def Gholdengo: 140-166 (44.4 - 52.6%) -- 20.7% chance to 2HKO (with item)
0 Atk Gliscor Knock Off vs. 0 HP / 0 Def Gholdengo: 94-112 (29.8 - 35.5%) -- 22.6% chance to 3HKO
252 SpA Gholdengo Make It Rain vs. 244 HP / 12 SpD Gliscor: 249-294 (70.7 - 83.5%) -- guaranteed 2HKO after Poison Heal (non boosted)
Heatran can trap it
0 SpA Heatran Magma Storm vs. 244 HP / 12 SpD Gliscor: 169-199 (48 - 56.5%) -- 88.3% chance to 2HKO after Poison Heal and trapping damage
Glimmora 252 SpA Glimmora Power Gem vs. 244 HP / 12 SpD Gliscor: 163-193 (46.3 - 54.8%) -- guaranteed 3HKO after Poison Heal
If not on toxic
Great Tusk 252 Atk Great Tusk Icicle Crash vs. 244 HP / 252+ Def Gliscor: 232-276 (65.9 - 78.4%) -- guaranteed 2HKO after Poison Heal
Dragonite 252+ Atk Dragonite Icicle Crash vs. 244 HP / 252+ Def Gliscor: 260-308 (73.8 - 87.5%) -- guaranteed 2HKO after Poison Heal
Ok, a final explanation. If they are bold they have recovery. If they are italicized they can set up on gliscor taking advantage of it. If they are underlined they can ohko gliscor. There are some checks/counters/switchins that are for specific sets. Do not say they do not work unless you are talking about the sets I am talking about. Otherwise we are arguing 2 different things and it is unproductive.
As a final takeaway:
Dozo, clef, torn can basically switch in to any set, pressure gliscor/set up and don't lose too much regardless of the set (also before you say but they can knock it, any knock off user can knock on a switch in. The fact that it can knock is not enough. Iron valiant, great tusk, ogerpon all can knock off you checks/counters item). All three of these mons also have recovery to help its longevity.
Alot of pokemon can pressure it once it is in. Manaphy, WW, slowking galar, ninetails aloa, greninja, and weavile. Of these slowking is of note since it has recovery to help match gliscors longevity.
And finally certain sets have some pokemon that hard answer them like dragonite (which has longevity because of roost) and tusk for the non toxic versions and air balloon heatran and gholdengo (ghold even gets recovery) for the non knock version.
There are more answers than just MG clef. Dozo for example is another good answer to gliscor. If you would like to discuss any of these I welcome the discussion, but please talk specifics instead of just saying a general "6 don't have longevity". Please lets have a real and productive conversation, instead of short 1 line posts that don't explain anything.