Note that I've reproduced today this very same issue with lightdm as well, so I guess this is the confirmation that the bug is somewhere else. Graphics drivers maybe?
- Bug#909770: gdm3: Does not show the list of users and ... Sergio Villar Senin
- Bug#909770: i915: Does not show gdm greeter on eD... Simon McVittie
- Bug#909770: i915: Does not show gdm greeter o... Sergio Villar Senin