GUI does not start

Initially, the computer was showing the error as mentioned in How to fix "The system is running in low-graphics mode" error? question. Fixing it resulted in the above error.

Please take look at the last line

Started GNOME display manager... and deal with any system changes..p link was shut down.....

One more thing before all this problem I have edited xorg.conf file to use Tooya X graphic tablet as suggested on here.

I don't know if that's the reason but I thought it is important to share.

Alt+F2 keystroke which started the terminal session for me and I was able to login.

sudo apt purge gdm gdm3
sudo apt install gdm3 ubuntu-desktop

Then choose your display manager in the menu that opens (for Ubuntu 16.10+ the default is GDM, for 16.04 and earlier the default is LightDM), or use sudo dpkg-reconfigure gdm3 to choose GDM.

systemctl restart gdm

This should work

There appears to be other people having this issue as well. A bug report was also made on this. It appears to be a bug with version 3.16. A temporary fix would be to use LightDM instead, so (in a VT, eg Ctrl+Alt+F1:

sudo apt-get install lightdm
sudo dpkg-reconfigure lightdm

Press enter to get past the "Configuring lightdm" screen, and then navigate to LightDM in the list of options and press Enter. Restart, and you should be using LightDM. This should work temporarily until the GDM bug is fixed.