#edit the file /etc/gdm/Init/Default
vi /etc/gdm/Init/Default
# search for
PATH="/usr/bin:$PATH"
OLD_IFS=$IFS
#add these lines after it:
xrandr --newmode "1392x768" 87.25 1392 1464 1608 1824 768 771 781 798 -hsync +vsync
xrandr --addmode VGA1 1392x768
xrandr --output VGA1 --mode 1388x768
#save