I’d have a couple of concerns with a metal roof on the Gravity if it ever became an option. One is headroom. The glass roof appears to have been an integral part of giving lots of headroom to the 2nd and especially 3rd row. Would a metal roof with a liner cut a couple inches of headroom? A second concern would be the feel of the 3rd row. The D pillar of the gravity (like the C pillar on Air) is really beefy. There is essentially no side window for the third row. I think with a lowered ceiling and no light, the third row would be a pretty crappy place to be - assuming you select that option and/or depending on how much of a use case the third row is for you.)
I think some of the interior features I’m referencing can be seen pretty clearly in the long OOS vehicle walkthrough posted a few days ago.
I have never used a fabric roof insert, so I can’t speak with any experience, but I wonder whether the huge size of the Gravity glass roof would lead to the center of such an insert sagging and still sacrificing headroom?
Admittedly I am not so concerned about the glass roof but I think that if I were, I’d put a dark tint on the glass before switching to a metal roof.