If it is about not everyone agreeing how to play then yes, It is not an issue to be solved within the game. I have said this many times. But these situation in practice are not about that, they're about the player having misunderstood or didn't know of something and the GM clarifying. It is literally no different than the player thinking that the orc was closer than it actually was.
Oh, okay, if you were just talking about making sure the players and the GM are on the same page about what's going on in the scene, we're in agreement -- this should happen. This doesn't have anything to do with "metagaming" though, as it's the GM updating the players' understanding (or vice versa) and not anything to do with characters -- except that the downstream effect would be that players can now direct their characters without the misunderstanding of what's happening in game. This doesn't appear to implicate the GM telling players what their character is allowed to think.
Okay, good, then we can dispense with worrying about a player knowing a trap in in a place or whether or not an NPC is lich, then, right? Because, fundamentally, the only problem with considering these poor roleplaying doesn't come from the player declaring their character thinks something, but that the player declares their player thinks something that is both true and uncovering something secret in the game. If these are not the issues, we can focus more tightly on those that remain. I will confess, though, that the only things I see that remains is an aesthetic desire for play to
look a certain way.
Certainly the physical possibility of gunbowder existing but knowledge of it not is trivially true for any pre-gunpowder historical setting? Or any setting that would be defined to be similar to one that this would still apply? Furthermore, don't get hung up on gunpowder, the same applies to many purely mechanical tasks where it it would be super hard to imagine how it would not work even in fictional setting. Ancient Romans most likely had required metallurgy to physically produce full plate, but they didn't.
Oh, I'm not hung up on gunpowder. Or on the other things you're vaguely waving at -- these aren't problems I have in my games, largely because I don't make the blanket assumption to start with that these things could exist exactly as they do in the real world. As I noted, if you do this as a GM, then you're already creating the problem you care about. Don't, and you won't. If you allow that gunpowder exists, but work to prevent any realization of gunpowder, then I'm not sure what your goal is, but it's not terribly coherent.
But, even if you do, you still get to adjudicate the actions necessary to bring these things to life. I'm fine with a player asserting that gunpowder can exist and it's a precise mixture of elements. I can also test any attempt by that PC to implement this via the resolution mechanics, or any attempt to tell others that it exists via the resolution mechanics. If I'm allowing for gunpowder to exist, I damn well better be onboard with PCs discovering/inventing/creating it.
Oh, and the thing about the GM knowledge is rather interesting. So if the GM doesn't know what the thing the PC is doing would logically result they can say it doesn't happen, but if they do know they can't? Whoa!
I know, right? So why is it a required assumption for your case? That's what you postulated -- that a PC declaration to invent gunpowder could be easily foiled by the GM, but a step-by-step delineation of the process in detail would prevent the GM from foiling it. I mean, the assumption that the GM would want to foil it is shaky, as I talk to above, but here you've clearly made it the case that the GM cannot know what the actual process is because the GM can't recognize or stop it until it's too late. This assumption is necessary because you've skipped any step where the GM can adjudicate an action, leaving the GM at the mercy of the methodological player. It's weird -- the GM has set that gunpowder exists exactly as in the real world, and you have the GM adjudicating as if they know the formula for your first case, but then set up the second where the GM is powerless in the face of the exact formula. It's just odd, I agree!
Yeah, GM can totally do that. Sometimes that is the most sensible course of action, sometimes it isn't. And sometimes some thoughts are effectively 'fictional positioning.' If no one, or at least their character. cannot know someting then the GM is fully within their rights to tell the player that.
I don't worry about it or 'police' it either because I don't have to. Players understand that their characters are people in fictional setting and that puts some limitations on their knowledge, not do their resent if Gm sometimes has to clarify their understanding. Similarly as a player I am merely thankful if the GM clarifies something I had misunderstood or assumed wrongly.
Oh. You meant clarifying the situation as the GM telling the player what their character is allowed to know. Yeah, I suppose we are in violent disagreement here, as well. I thought you meant clarifying things like how far the orc is from the PC, which is a player confusion, not a PC confusion, but you mean something more, like telling the player that their PC doesn't even know it's an orc, so they can't call it an orc. Yeah, good on that first bit, not good on the second.
All it seems to be here is that you've taken monitoring and controlling PC play and lumped it into clarifying the flow of game information between GM and player. This is a category error, in that the kinds of information flowing here aren't the same. One is physical information about the game world, the other is the GM telling the player how to play their character.
I have playd a lot of other games, freeformed, LARPed and written my own homebrew games and then ran them. D&D is probably not among my most played games. This is really not just about the D&D.
My apologies, then. It's unusual, in my experience, to encounter someone with a lot of experience in playing different games that also insists that you use rules and understandings from one game in another. You've been very consistent in applying previous edition thinking to 5e, so it seemed that you might lack a wider experience.