Assuming you are using Raspbian Jessie, here is 2 steps to have your Pi booting with GUI as root.
edit the file: /lib/systemd/system/getty@.service
ExecStart=-/sbin/agetty --noclear %I $TERM
ExecStart=-/sbin/agetty/ --noclear -a root %I $TERM
Remember to Save.
edit the file: /etc/lightdm/lightdm.conf
(make sure you remove the “#” !)
Save and Reboot.
NOTES: You must know that this is a hackers dream to find your open autologin root system. But roboticists, home dev types and air gapped cnc machinists I am sure are sick of everyone keeping this knowledge in pieces or behind protective walls. I am posting this for myself because it seems harder to find it each time I need it. I DONT KNOW If step one is required. I will test it some day when I build another system.
I just tested this on Rpi2 with 2.8 PiTFT capacitive touch screen. Good luck on your creations… I don’t usually make technical posts but I see too many struggling to remain silent.