I see this question is "answered" by stating that there is no way to do this in GNOME: How to move windows around and between monitors, using keyboard short cuts? I have doubts that it should be impossible and I really want to do it.
Is there really no way to do this is Gnome? This is my output from xrandr that I don't know how to use:
$ xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 3840 x 1200, current 3840 x 1200, maximum 3840 x 1200
default connected 3840x1200+0+0 0mm x 0mm
3840x1200 50.0*