What about Safety Goggles? Is it that useful to be immune to sandstorm and hail damage? People still use it only because of Spore. If Spore is not broken nor shall STag be, as you can run Shed Shell.
Gengar user cannot be safe as it can be hit by Magic Bounce users as well. How will the opponent know whether u had Magic Bounce on this pokemon?
These are just some thoughts that can almost for sure counter Gengarite.
Quick edit: One or two Safety Goggles in team is enough to counter Spore. Spore users can always bring knock off. On the other hand, not all walls need Shed Shell as you can simply bring a move to kill Gengar (such as the common Spectral Thief and Moongeist Beam).
Uh... Spore can be dealt with by other methods too besides just an item? You can run Poison Heal, which is already a good ability as is, Comatose, which is Poison Heal's brother that frees up an item slot instead of providing healing but lets you be immune to a few other nice things, and Magic Bounce as you brought up when dealing with Shadow Tag, to name the more common ones. If you really want you can be niche with your choices (Electric Terraiiinnnnn) or run some other item (Chesto/Lum Berry, to name 2) but you have a lot more options to deal with Spore.
With Shadow Tag, and to a lesser extent because of its Steel focus Magnet Pull, the only thing you can do is run a few certain abilities, ghost types, an ability suppressor, or 1 practically useless item and even then it's not guaranteed you can deal with it. Magic Bounce sets can be dealt with just by running the appropriate coverage move on Gar, anyway. You can even run Imprison + (switch move) if you really wanted to on Gar but I don't recommend this. Some teams I've seen also run another Gar alongside it, so on those teams you can't even risk sending the ghost in on a switch. Ability suppressors require prediction which both players can get wrong. Hell, Magic Bounce sets in general can still be trapped and killed as shown by the replay with me vs
Quantum Tesseract (I did it twice, the first on a Yveltal of all things and the second requiring me to sac MGar to get rid of the Imposter chans copying my M-Aud, but it was worth it), so saying that things can "almost for sure counter Gengarite" is really not a good idea. There aren't any true counters in BH for the strong threats like this, please keep in mind that anything can run almost
anything. And since you can't really switch out of Gengarite to go to your dedicated counter... the strict definition of a counter doesn't really apply to it regardless.
So what about Spectral Thief and Moongeist Beam? They're common after all, and super effective, so MGar should definitely be taken down if you just click them against it, right?
252 Atk Zygarde-Complete Spectral Thief vs. 248 HP / 252 Def Gengar-Mega: 150-178 (46.4 - 55.1%) -- 63.7% chance to 2HKO
Realistically, this would be the strongest Spectral Thief that can be used against a Mega-Gengar by a wall, and yes, it can 2HKO, but if Gar's running +Def (mines does for this explicit reason) this is a non-issue.
252 Atk Zygarde-Complete Spectral Thief vs. 248 HP / 252+ Def Gengar-Mega: 136-162 (42.1 - 50.1%) -- 0.4% chance to 2HKO
And remember, this is the strongest one that would realistically be used in this situation, because Zygod has the highest Attack out of the walls you'd send Gar in on. And it needs 2 high rolls to 2HKO. Say Gar has recovery (mines does). What then? You're gonna get locked and waste your Spectral Thief PP, only to run out of it and get Perish Song'd as you would've before. This gets even worse with Strength Sap because your Attack gets dropped, making Spectral Thief even weaker. Sure you drained a lot of Encore's PP too, but that doesn't mean Gar still can't pick off your wall. Of course, this is also a non-issue if you don't click this move in the first place, Gar just locks you into whatever else and picks you off. Really, the one wall that would be the most threatening is Gyarados-M simply because of its offensive capabilities and Knock Off being as strong as it is + getting STAB, but that's it, honestly.
Also, who runs Moongeist Beam on a wall? It wouldn't do much better anyway and because of its lesser PP it'd probably be much easier to stall out...