Choosing Shut Down from gdm login screen halts the computer, with the message Power Down. On ATX computers it should stop it completly, as in RHL 8.0. I'm not sure this belongs to gdm.
This is a duplicate of Bug 80360. Close this report as duplicate and provide your hardware info within that report.
*** This bug has been marked as a duplicate of 80360 ***
Changed to 'CLOSED' state since 'RESOLVED' has been deprecated.