The Lock signal is not emitted by systemd-logind when the screen is locked
Since Gnome depends on Systemd, it should be compatible with all Systemd components. According to the systemd-logind documentation the Lock signal will be sent to D-Bus by systemd-logind when the screen is locked. This does not happen when the lock screen is activated.
Compatibility is important for 3rd-party software to be able to react to the session lock event without depending on Gnome components (in particular the ActiveChanged event sent by org.gnome.ScreenSaver).
How to reproduce
Make sure that automatic screen locking is enabled.
Start monitoring D-Bus for the Lock signal.
sudo dbus-monitor --system "type='signal',interface='org.freedesktop.login1.Session',member='Lock'"
Lock the screen by pressing
Super + Lkey combination.
Unlock the screen and check the console output.
The monitoring tool should show a captured signal like below.
signal time=1541358211.693950 sender=:1.9 -> destination=(null destination) serial=2008 path=/org/freedesktop/login1/session/_36; interface=org.freedesktop.login1.Session; member=Lock
The Lock signal is not recorded by the monitoring tool.
When the screen is locked by a Systemd provided tool loginctl, like
sudo loginctl lock-sessions --no-ask-password, then the signal is emitted as expected.
Interestingly the Unlock signal is emitted, but that's because GDM calls the UnlockSession() method of the logind session object.