Comment 1 for bug 1819920

Unfortunately this is the default power button behaviour.

Even though we have

  Settings > Power > When the Power Button is Pressed = Power Off

to gnome-shell "Power Off" just means to show the dialog.

This is confusing, and something I have complained about myself. But if you wait 60 seconds without selecting anything in the dialog then it will eventually power off. Just as you asked.

If you want an faster system power off then try: sudo poweroff

The Settings GUI option of "Power Off" is actually just an illusion presented by gnome-control-center. The actual list of options supported internally is:

  <enum id="org.gnome.settings-daemon.GsdPowerButtonActionType">
    <value nick="nothing" value="0"/>
    <value nick="suspend" value="1"/>
    <value nick="hibernate" value="2"/>
    <value nick="interactive" value="3"/>
  </enum>

So there is no "Power Off" option in gnome-shell in reality. What you are seeing is the "interactive" option.

For gnome-session-quit to offer the "--power-off" parameter sounds like a promise it can't keep. Gnome only supports the four options listed above.

Similarly,

  gnome-session-quit --power-off --no-prompt

still prompts the user :P