gnus-use-frames-on-any-display

gnus-use-frames-on-any-display is a variable defined in `gnus-win.el'.
Its value is

nil

Documentation:
*If non-nil, frames on all displays will be considered usable by Gnus.
When nil, only frames on the same display as the selected frame will be
used to display Gnus windows.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.