Comment 7 for bug 1355711

Tom is right, gnome-session does uses systemd-logon's poweroff.

If you want to see the chain of events in gnome-session:
1. http://bazaar.launchpad.net/~ubuntu-branches/ubuntu/utopic/gnome-session/utopic/view/head:/gnome-session/gsm-systemd.c at line 446
2. http://bazaar.launchpad.net/~ubuntu-branches/ubuntu/utopic/systemd/utopic/view/head:/src/login/logind-dbus.c at line 2132