I... uh... I'm actually struck momentarily speechless by this claim. You're saying that what's happening in the game is an arbitrary division from some other, non-game thing, that you want to be part of the game, but not part of the game? I can't even follow this.
Gamestate is just shorthand for holistically referring to what's going on in the game. However hard you imagine what your character is thinking, though, this isn't part of the game until you actualize it in the game. You may think you're playing the game, but you're just imagining things (literally). The game doesn't care, nor does anyone else, until and unless you introduce it to the game. That's the only place where you can then evaluate agency -- does your introduction result in agency for you, the player? Most of the time, it doesn't. I mean, talking in funny voices with the other players is hella fun, I love it, but it doesn't do anything agency wise within the game -- it's, in fact, a meta-game of entertaining your friends, which you can do with or without the RPG you're playing. It's just freeform roleplay, for the most part. It only impact agency in the game when you provide an action that the game can operate on. I love acting out characters, but this isn't agency -- the game certainly doesn't allow or disallow it. Just like the Monopoly discussion earlier -- a funny voice and some characterization does not add agency to Monopoly.