Outside SD Gliscor, set up Garg has very few problems with it as boosting sets just set up in Gliscor's face, even denying the bat poison heal recovery (and pushing progress if rocks are up). Sub+ID Zama is a worse match up but it's also not hard to cover with something like tera ghost, or a partner like Zapdos, Moltres or a Toxapex for hazing boosts away. Garg also is a great spikes abuser due to the way it handles most spike immune mons well, and is itself still a great check to the likes of offensive Zapdos, TornadusT, Heatran, Volcarona, and physical attackers like Gouging Fire. And still offers respite against some threats like certain Koko variants as well. I don't really agree much has changed to make it much worse, let alone enough to drop it into B+ from A. Especially as, while sometimes wanting tera a lot, that tera usage can let the user cover some match ups otherwise hard to handle for a team. Like you may end up weaker to rain teams, and tera water/dragon Garg can greatly alleviate this weakness with not much help needed beyond maybe a back of check in case of emergencies.
A --> B+
Garg just struggles to do much in this Gliscor-infested world, and although I'll agree removing Poison Heal healing is a nice trait to have, it just doesn't feel as splashable as it did before Gliscor propagated the metagame. It also struggles against HO teams. I'll say its IDBP set is pretty decent as well as being a Rocks setter on bulky teams, but I just struggle to fit it a lot nowadays with how tera-reliant it is and how vulnerable it is to meta staples like the aforementioned Gliscor as well as Sub + IDBP Zama.