GTK+ 3.24.20Description by the Software Developer:
"GTK+ is a multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK+ is suitable for projects ranging from small one-off projects to complete application suites.
GTK+ is free software and part of the GNU Project. However, the licensing terms for GTK+, the GNU LGPL, allow it to be used by all developers, including those developing proprietary software, without any license fees or royalties."