Steam Login Session at Ubuntu 18.04
steam-login didn’t work for me. Instead I just did:
sudo apt install openbox
echo "steam -bigpicture &" >> ~/.config/openbox/autostart
and reboot (for GDM to realise there is a new option).
Note: The new session is called Openbox, but you can change that to Steam by editing /usr/share/xsessions/openbox.desktop.