How to start a X-application on remote server over ssh

It's right there in the last line:

debug1: Remote: No xauth program; cannot forward with spoofing.

In addition to gedit, you'll also need to install an xauth package. If the Ubuntu server wasn't installed with an X Windows system (probable considering you had to manually install Gedit) this is clearly not present.

Per dbus-launch(1):

If DBUS_SESSION_BUS_ADDRESS is not set for a process that tries to use D-Bus, by default the process will attempt to invoke dbus-launch with the --autolaunch option to start up a new session bus or find the existing bus address on the X display or in a file in ~/.dbus/session-bus/

Whenever an autolaunch occurs, the application that had to start a new bus will be in its own little world; it can effectively end up starting a whole new session if it tries to use a lot of bus services. This can be suboptimal or even totally broken, depending on the app and what it tries to do.

There are two common reasons for autolaunch. One is ssh to a remote machine.

So it seems the trick is to start dbus-daemon preemptively, in such a way that programs can find it. I use:

[me@host ~]$ dbus-launch --exit-with-session gnome-terminal

which, aside from gnome-terminal, starts dbus-daemon and sets $DBUS_SESSION_BUS_ADDRESS within gnome-terminal.

Any X programs run from gnome-terminal then behave nicely, and dbus-launch cleans up after itself when gnome-terminal exits.