Bug#724731: [gdm3] gdm does not start properly, cannot login via gdm

[Jascha Geerds]

Did you only installed "libpam-systemd" or did you also boot with
systemd (instead of SysVinit) ?

You can temporary boot using systemd by editing the GRUB kernel command
line (press "e" inside GRUB) and append "init=/lib/systemd/systemd" to
it. For further information see https://wiki.debian.org/systemd

-- 
  Jascha Geerds
  jg at ekby.de