I’ve seen a lot of people say “Gorilla Tactics isn’t OP by itself so G-Darm must be the problem; ban G-Darm.” Let me present a hypothetical scenario in response:
Let’s distribute Shadow Tag to a bunch more Pokemon: Caterpie, Weedle, Ledyba, Snom, Magikarp, Unown, Feebas, and a bunch more super weak mons.
In this scenario, do we ban Shadow Tag? I don’t think so.
Shadow Tag was banned because it’s broken/uncompetitive on all its users, so banning the ability itself was a simple solution (that also preserves other Gothitelle variants). But in our hypothetical world, Shadow Tag by itself obviously isn’t the problem, and banning it also prevents a bunch of obviously weak Pokemon from using the ability. Given that the OU Council prefers to avoid complex bans, including banning specific Pokemon/ability combinations, I imagine we would have ended up banning the Gothitelle line and Wobbuffet.
Gorilla Tactics is in a state similar to Shadow Tag: it might not be OP if given to weaker Pokemon, but it hasn’t been, so it’s broken on all its users. In fact, I'd actually argue that a G-Darm ban would be inconsistent with the choice to ban Shadow Tag rather than Gothitelle and Wobbuffet.
One could argue that Game Freak could introduce a new Pokemon with Gorilla Tactics, and that we wouldn’t want to ban that Pokemon prematurely. But GF could also distribute Shadow Tag to a bunch of weak Pokemon in the next Gen 8 games, and we’ve banned Shadow Tag. Really, “Gorilla Tactics wouldn’t be broken on another Pokemon” is just as much of a hypothetical as Shadow Tag Caterpie.
Anyway, if we ban Gorilla Tactics we can: a) get rid of the problem, b) preserve Zen Mode G-Darm, and c) stay consistent with the Shadow Tag ban. If GF does introduce a non-broken Gorilla Tactics user later, we can then ban G-Darm the Pokemon.
(Also, I wrote this post before I saw DialgaTheTimeLord's post so here's a few points in response.
a) I'll admit I didn't realize Shadow Tag was that brutal on the Gothitelle/Wobbuffet lines in the past, but I think Rarycaris's point that you can't separate the Pokemon from the ability still stands. Sure, some of them have garbage BSTs and are trash without Shadow Tag, but... these are Pokemon that are literally designed around abusing the ability, with moves like Encore/Mirror Coat/Counter/Destiny Bond/Trick/Taunt, etc.
So really, is it the ability that's uncompetitive, or is it the ability plus specific sets of moves that all the Pokemon with the ability happen to have that are broken? If it's the latter, that seems to me like exactly what Rarycaris was getting at.
b) Why isn't Arena Trap banned? Sure, it's weaker than Shadow Tag, but doesn't it still have the same aspect of preventing switching? Seems to me like the fact that Dugtrio doesn't have Encore/Taunt/Trick weakens and changes the ability substantially, so it can't pull off the same uncompetitive shenanigans as Gothitelle. Hence, you can't separate the Pokemon from the ability.
c) Why was Shadow Tag banned instead of the Gothitelle and Wobbuffet lines? Why shouldn't we err on the side of banning the ability alone when there won't be any collateral damage from doing so?
d) Shadow Tag is obviously way more powerful than Gorilla Tactics. Just noting it here so people don't pile on me for thinking they're equivalent in power or anything.)