I pulled the interior roof skin off of my BB and measured it at about .0310 or .0315 (22 gauge?).
I think this will work out awsome for window skin. I know it is a little on the light side and I need to check my measurments to be sure I have enough but I think it will work. The stuff from the walls is to thin but the ceiling is a little better. Most people seem to prefer 18 or 20. If I have to add a cross brace horizontally in the middle of each window section and tack or rivet a few times that is worth the 2 or 300 in steel. Plus it is pre painted so rust shouldnt be a problem right away.
If I was buying it I would go a little heavier but for free you cant beat it. Besides I think it is the same guage as the rest of the exterior (correct me if I'm wrong I havent measured.