Even more interesting is an upstream gnome-bluetooth bug report [1]. As I understand it, the pulseaudio module module-bluetooth-discover [2] is responsible for detecting available bluetooth audio devices using BlueZ. GDM starts a pulseaudio process [3] before the user does and module-bluetooth-discover in fact works - but for the GDM pulseaudio process, not the one that gets started with X11.
I verified that by connecting my bluetooth speakers in GDM (I can connect to the last used paired device with a button). After connecting the speakers, GDM used them for audio output immediately. When I comment out module-bluetooth-discover in /etc/pulse/default.pa as described in Comment 1 in the upstream gnome-bluetooth bug report [1] and then add it to /usr/bin/start-pulseaudio-x11, the bluetooth speakers get detected without any problems and are listed in gnome- control-center audio settings. So in my view this is no BlueZ bug. I think it would make sense to reassign it to PulseAudio. [1] https://bugzilla.gnome.org/show_bug.cgi?id=749208 [2] https://wiki.freedesktop.org/www/Software/PulseAudio/Documentation/ User/Modules/#index35h3 [3] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=805414
