Why can't I run Gnome apps over remote SSH session?

Indeed when an SSH session is open, it doesn't launch a dbus session. Some programs may launch it, but then the session doesn't know about it (hence can't close it).

Not knowing about the dbus session also means that programs that use dbus but don't launch it themselves will have problems.

dbus sections are per machine and per X11 display. Their info is stored in $HOME/.dbus/session-bus/- however, the process referenced there may be closed, so an extra check is needed to determine if launching dbus is needed or not. Then, the variables there are to be exported to the session.

Then it works like a charm :)

I put the following in my .bash_profile file:

# set dbus for remote SSH connections
if [ -n "$SSH_CLIENT" -a -n "$DISPLAY" ]; then
    machine_id=$(LANGUAGE=C hostnamectl|grep 'Machine ID:'| sed 's/^.*: //')
    x_display=$(echo $DISPLAY|sed 's/^.*:\([0-9]\+\)\(\.[0-9]\+\)*$/\1/')
    dbus_session_file="$HOME/.dbus/session-bus/${machine_id}-${x_display}"
    if [ -r "$dbus_session_file" ]; then
            export $(grep '^DBUS.*=' "$dbus_session_file")
            # check if PID still running, if not launch dbus
            ps $DBUS_SESSION_BUS_PID | tail -1 | grep dbus-daemon >& /dev/null
            [ "$?" != "0" ] && export $(dbus-launch) >& /dev/null
    else
            export $(dbus-launch) >& /dev/null
    fi
fi

notes: hostnamectl is part of systemd and allows to retrieve the machine-id the dbus-launch displays the variables we want; by using export $(dbus-launch) we retrieve the output of dbus-launch and export the variables

None of the previous answers worked in my case but launching the application through dbus-launch did the job:

ssh myhost "dbus-launch gnome-terminal --display localhost:10.0 &"

I found this:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=639261

Which led me to try this:

$ sudo rm /var/lib/dbus/machine-id
$ sudo service messagebus restart

Now I can run gnome-terminal!