gdm 3.29.90 fails to start: Failed to set CRTC mode 1024x768: Permission denied
I don't have much information to go on yet, but with the Debian/Ubuntu packages (tried on both) of gdm / gnome-shell / mutter 3.29.90, GDM most of the time doesn't start. I did get to see it once or twice, so this seems like a race.
plymouth is not installed.
@halfline told me on IRC to try booting with
drm.debug=0x1e. Here's the journal after it happened with that in the commandline.