Running the program with the "–xboard" command line parameter sets it to produce output acceptable to and accept input suitable for XBoard and WinBoard, the graphical display front-ends with mouse interface.
For historical reasons the option "xboard" does not need to be preceeded by "–", however we would encourage the new syntax.
How to run XBoard with GNU Chess as chess backend:
xboard -fcp 'gnuchess --xboard' xboard -fd . -fcp './gnuchess --xboard'