So I have been looking into Gliscor counterplay for a bit. I really don't think it is broken. The main thing is the passive and SD sets have somewhat different counterplay. I feel like the passive sets have been covered pretty well on here. But getting other people to talk about the SD set counterplay in this thread has been somewhat tricky. With a Gliscor suspect looming, I will attempt to get the conversation started again. I will leave out obvious mons like Corv and Dozo for the time being.
SD sets can really only fit two moves, usually between Facade, Knock Off, and EQ. Outside of the metal birds, there are also a decent amount of mons that can resist 2 of the 3 moves like Sinistcha, Gambit, Tinkaton, or even less seen niche picks like Mandibuzz or Tyranitar. Though I have also seen Ice Fang and EQ.
The thing about Ice Fang is it also has a low base power. So it doesn't tend do much damage outside of supereffective hits on SD boosts. Knock Off has a similar problem, not hitting as hard after the first hit. You don't necesarrily need to have a straight resist to it. EQ can be mitigated by Grassy Terrain and/or Air Balloon. EQ also has a lower PP, so it can more easily be stalled out. What I'm saying is there are various way to play around EQ.
One mon I want to bring up is Garganacl. You resist Facade, you counter Poison Heal recovery with Salt Cure, and you are immune to Toxic. After the first Knock Off, it wouldn't do that much. You can boost your own defense with Curse or Iron Defense. I feel like this is a pretty good mon into Gliscor if you have a way to mitigate the threat of potential EQ. You can also make your Tera with this in mind.
Another mon I have been experimenting with is Orthworm. It's... interesting. I don't know if it is good in OU. But you are immune to Toxic and EQ, you resist Facade, and you have a naturally high 145 base defense. I also considered it for Kyurem because you resist both the STABs while being immune to Earth Power. It feels like there is an anti-meta mon in there somewhere. No recovery hurts it, but you can maybe run it with Wish support on more defensive teams. While more testing is needed, I wanted to mention it now before the Gliscor suspect.