To login a certain user automatically at boot time, you have to change the daemon-section of your /etc/gdm/custom.conf:

[daemon]
TimedLoginEnable=true
TimedLogin=you
TimedLoginDelay=0 

With ‘TimedLogin’ being a valid, non-root user and ‘TimedLoginDelay’ being the login delay in seconds.

An overview of available configuration variables can be found on live.gnome.org