Cannot use gdbus command against same arguments of dbus-send

[Germano Massullo]

Hello, on Fedora GNOME Xorg session, I am using

    $ dbus-send --print-reply --dest=org.gnome.Mutter.IdleMonitor
/org/gnome/Mutter/IdleMonitor/Core
org.gnome.Mutter.IdleMonitor.GetIdletime

to retrieve the user idle time. Indeed I get message

    method return time=1558965539.579835 sender=:1.19 ->
destination=:1.97 serial=409 reply_serial=2
       uint64 17

But if I try to use gdbus command

    $ gdbus call --system --dest org.gnome.Mutter.IdleMonitor
--object-path /org/gnome/Mutter/IdleMonitor/Core --method
org.gnome.Mutter.IdleMonitor.GetIdletime

I get the error

    Error: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The
name is not activatable

Why I am not able to use gdbus command against the same arguments?
Thank you