Here's some concrete examples.
I do write short stories, very small amount of poetry, one story long enough to be a novella. I enjoy it.
When I am roleplaying, let's say Cormac the warrior, I'm trying as much as possible to think like Cormac, do what he would do, and say what he would say. Is there some crossover there in coming up with a literary character's actions and dialogue, perhaps, but in actually WRITING fiction I am thinking about composition, plot, syntax, all the things that go into literary creation. When I am roleplaying, I am LIVING fiction and I think of none of that. I simply react as Cormac.
Later, still roleplaying, Cormac is telling of his deeds to another clan. At that point I am creating a story, as Cormac. I'm deciding, as Cormac, how much to tell, how much to leave out, what to embellish, etc. At that point, the artistic creation begins. There was no artistic creation and no story prior to that. None. There were simply events and facts that occurred. Fictional events and facts, yes, but not the same type of fiction as the later story.
I write creatively.
I GM all the time.
When I roleplay, I want what I can't get from those other activities, and see no need to blend or mix them because I do all three. Doing all three frequently, I know exactly what I get out of them and how I think when I do them.
They are three different things.