Defensive checks:

(Bulk Up + Brave Bird): ID Body Press gets PP stalled by Gliscor, particularly if paired with a Ghost type, so you need the Bulk Up Bird. But it’s worth noting that if Gliscor is paired with rocks and a good Corv counter like Ghold, it can simply knock Corv and wear down its limited roost PPs over the game.

ID checks most SD Glisc sets once per game. Keep in mind that in this interaction, Gliscor will knock your lefties, get some chip on you, then freely switch to a hardcounter like physdef Ghold or Sinistcha while you’re left with a crippled Zama for the rest of the game. So Zama only really functions as a late game emergency check if Gliscor’s ghost teammates are gone.

Beats standard SD Gliscor with Curse or Body Press + Avalanche. However it will be taking a knock, which makes it quite exploitable given that Gliscor is typically paired with hazards.
Set specific defensive checks exist, such as

for Knock Facade, but this requires scouting.
Offensive checks:

The most notable offensive check since it’s quite strong against the general team structures that SD Glisc finds itself on. Consequently, the best play for these teams is often to simply commit Tera Normal to take out Kyurem.

Beyond strong Water STAB, it can pack Encore to make things annoying for Gliscor.

Not as common as the former two but it’s an option (if you’re cool).
Niche stuff:

(Encore): Physdef Clef is only 3HKO’d by +2 Facade, doesn’t worry about Knock, and can Encore Gliscor to give its team some breathing room while scouting the Gliscor set. Probably a good tech in the Glisc meta.
Overall it seems that Gliscor’s most splashable checks lack the longevity to handle it long term. Its ability to cripple mons like Zama and Corv with Knock and outlast them is notable. I personally feel it’s worth a suspect.