If you by mistake choose during Ubuntu / Xubuntu install the user to have password and later you want to change the default GDM login behavior to not ask for password you can do it slightly easy by simply adding the user you like to login without any password prompt in /etc/group – nopasswdlogin group.
# grep -i nopasswdlogin /etc/group
Edit the file with mcedit, vim, nano whatever text editor and add the user to it.
Let’s say you would like to add the testuser to login in GDM (Gnome Display Manager) with no password the line record should be changed to:
No need to tamper with gnome-control-center (User Registrations) menu, neither to edit complex GDM config files it is as simple as that 🙂 Restart gdm and on next login clicking over testuser avatar will log you in ur Desktop 🙂
Enjoy ! 🙂