I have been using stretch now for a week. I believe that it seems GDM3 is not reading any config files I place in /var/lib/gdm3/.config.
In this directory I have placed a monitors.xml that specifies which monitor is primary. Furthermore I configured GDM3 not to capture a bluetooth ad2p sink (as described in the wiki: https://wiki.debian.org/BluetoothUser/a2dp).
Now it seems that still GDM3 captures the ad2p sink (because I can't get bluetooth to work), and the monitor I specified as primary in monitors.xml is not made primary. Hence I believe GDM3 is not reading any config files in /var/lib/gdm3/.config.
I checked the file permissions on the directories and files, which should be okay; both the user and group are Debian-gdm, and files have rw permissions, and directories rwx permissions.
I know for a fact that my bluetooth a2dp sink is working properly, because if I use lightdm there are no issues with bluetooth.
Any ideas if I am doing something wrong, or might there be a bug somewhere in GDM 3.22? Is there a major change between 3.22, and 3.14 (3.14 is the GDM version in Jessie) that could explain these two issues?