I don't mean to single you out here -- you just happened to make the first post on this.
The thing about signature abilities, especially ones which are not the Pokemon's only ability, is that you can't as easily separate the broken user from the broken ability. Shadow Tag is broken, but it wouldn't be broken on a 30BST Pokemon which only learns Splash. It is, however, broken on multiple users, to the degree that it benefits the metagame to remove the single common broken element.
I would argue that it's the same deal here -- the question's practical effect, however we formulate it, is "do we want to ban Zen Mode Galarian Darmanitan or not?". It seems bizarre to me that the answer to that question changes depending on how it is phrased, even if its actual semantic content doesn't change. Addressing the question directly, I see nothing to suggest that Zen Mode Darmanitan-G is broken. Crucially, banning Gorilla Tactics instead of Darmanitan is not a complex ban in the same way that, say "no Speed Boost on Blaziken" would be. Gorilla Tactics plus Darmanitan can be perceived as a complex ban, but in practice it isn't because it's the same as "ban Gorilla Tactics". I contend that any complex ban capable of being expressed as a simple ban is not a complex ban at all.
Or to put it another way, an ability's distribution is a trait of that ability, just as much as its effect is. Nothing that gets Huge Power is broken with it, even though the ability in a vacuum is objectively better than Gorilla Tactics. This isn't Hackmons; there's no reason why we are obligated to regard an ability's effects only in a vacuum. Gorilla Tactics has better distribution, and is thus in my opinion a better/broken ability. There's historical precedent for this, in that metas have existed where Arena Trap was banned but Shadow Tag (a strictly better ability in terms of effect) was not, because Arena Trap's distribution made it more of a problem.
This is a HEAVILY flawed way to look at it. The reason why Gorilla Tactics itself isn't inherently broken is because, like Huge Power before it, it's merely a power increase, only this time with
detrimental effects to go with it. Huge Power is only "broken" in the case that the Pokemon it boosts has an already too high base Atk and tons of other stuff going for it. Mega Mawile was fine in gen 7 because the meta was strong enough to deal with it and the Pokemon itself was frail and lacked good STAB. The ability skyrocketed its Atk to an effective base 257 with max investment, but that alone wasn't enough to break it because it was slow and had no ability to hold additional items. Azumarill wasn't broken with Huge Power either, as its base Atk without it left something to be desired, so said ability brought it up to a solid level. Nothing inherently broken about that. It effectively just gave it a better Atk stat and that was it. Same goes for Gorilla Tactics, except now you can only reasonably carry Choice Scarf/Band, severely limiting your item options, while
also being way less powerful than Huge Power by comparison, at only 1.5x vs 2x. By all intents and purposes, it is
strictly worse than an ability that has zero reason to be banned.
Shadow Tag is
entirely different. Your example is also a straw man. Obviously the ability isn't broken on a Pokemon with a 30 BST and only knows Splash. That's an ABSURD counterargument. Short of an ability that KOs the opponent upon entry, nothing could make such a Pokemon broken. Yet may I remind you that Wynaut, a Pokemon with 93/48/48 bulk and at the time literally only four moves, was banned to Ubers
before Eviolite even existed. Shadow Tag is inherently broken because it directly impacts gameplay by eliminating a fundamental core aspect of the battle: switching. This makes it
inherently noncompetitive. Even against
Gothita, theoretically speaking, if it just so happened that Gothita could hard counter the right Pokemon in OU, there would be literally nothing you could do about it. And if you plan to suggest that the meta should just "adapt" to that, may I remind you we're talking about an
LC Pokemon here. And if this sounds farfetched to you, let me remind you that Gothorita was definitively banned in Ubers, and that was before it even got Cosmic Power + Nasty Plot + Stored Power. But obviously without said ability, neither Gothitelle nor Wobuffet, let alone any of their previous stages, are anywhere near Ubers, or even OU. Goth was PU last gen after losing its ability.
It went from Ubers to PU solely due to the loss of Shadow Tag. I CANNOT stress enough that Shadow Tag is fundamentally flawed, whereas Gorilla Tactics is just good and fundamentally sound, but too good when paired with every other good thing Galar Darmanitan has.
EDIT: also thanks, DrPumpkinz. My thoughts exactly.