Thanks! I read the thread where Dr. Toole was saying that GoM was not acoustically transparent, and it is interesting information. As speaker cloth, I can see where it would definitely "muffle" the highs to some degree, especially the anchorage fabric which has the lowest NRC rating of pretty much all the AT fabrics that GoM makes (.85)
However, losing a few db at the high end is something that equalization should be able to easily account for just like with an AT screen that shows similar rolloff at higher frequencies. I am not terribly concerned, but I am taking measures so that the GoM will only cover the surround speakers. I will be replacing the Onyx GoM Anchorage fabric planned for the fabric around the screen with something like DMD fabric, which I have used in the past and should work just fine. But that is easy because it is black. There are few colors made in the DMD fabric, but aside from the black none that I am interested in. I don't want the old school purple burgandy, and I don't want fire engine red, I am looking for more of the Mulberry color in the Anchorage fabric. I made a design change to the ceiling as well that will allow me to use basic black grill cloth over the Atmos speakers if I find the GoM is causing any issues.
I only wish there was a little bit of stretchiness in the GoM Anchorage.. I am worried about getting it tensioned right and no having it sag, especially on the ceiling panels.