The main units are started in the following order:
 * graphical-session-pre.target (systemd special)
 * gnome-session-pre.target
 * gnome-session-initialized.target
     Signals gnome-session-binary using gnome-session-signal-init.service
 * gnome-session.target
 * graphical-session.target (systemd special)

In addition, there are the following special units:
 * gnome-session-stable.timer and gnome-session-stable.target
     Can be used to modify failure behaviour when the session is
     considered stable
 * gnome-session-shutdown.target
     Should be started with replace-irreversibly when the session should
     be shut down; mostly useful internally by gnome-session with
     OnFailure/OnFailureJobMode
 * gnome-session-failed.target
     Should be start when an important session component failed. This
     can be used for OnFailure

The following additional units can be used for depenencies/conflicts:
 * gnome-session@.target
     To depend/conflict a specific session type...