I finally got an image of this gall maker. I was taking pictures of the pupa which I had just removed from a bunch gall collected on September 18. It was very restless; suddenly my annoyance turned into delight and amazement when I realized that the adult was trying to get out of the pupal case. Better yet, I had just replaced the batteries, so I kept the video camera running. the whole process took less than 5 minutes. Here is the video.
I guess that the recently emerged adult pumps some air so it becomes larger than the pupal case it just emerged from.
Dr. Netta Dorchin confirmed the ID of this individual.

More on goldenrod galls fauna.
Higher res. image