Who had the idea that Kyle Gibson was going to be called up this season? As far as I can tell, it was an invention of the media blogsphere. No one with the Twins was saying anything like that on the record and I doubt they said it off the record either.
It was obvious from the start of the season that a callup was unlikely unless something extraordinary happened. The Twins rotation was already six deep with proven major league starters. Regardless of how Gibson pitches at Rochester, he is likely going to have a learning curve once he gets to the big leagues. The middle of a pennant race is hardly the point at which you start that process.
They don't need to add him to the roster this fall. What possible purpose would it serve to call him up unless he is absolutely dominating every hitter he faces. He didn't do that last year even at lower levels. He is a solid prospect and will likely nail down a rotation spot at some point. But that happening this year was always very unlikely.