libgdm1-dbgsym 3.16.2-0ubuntu2 (amd64 binary) in ubuntu xenial

 GDM provides the equivalent of a "login:" prompt for X displays: it
 asks for a login and starts X sessions.
 .
 It provides all the functionality of XDM, including XDMCP support for
 managing remote displays, and extends it with the ability to start X
 servers on demand.
 .
 The greeter is written using the GNOME libraries and hence looks like a
 GNOME application - even to the extent of supporting themes!
 .
 This package contains the shared library required by gnome-shell.