Output selected images as .ggr gradient files (GIMP).
If no gradient name is specified, it is deduced from the filename.
G'MIC is an open-source software distributed under the
CeCILL free software licenses (LGPL-like and/or
Copyrights (C) Since July 2008,
David Tschumperlé - GREYC UMR CNRS 6072, Image Team.