gtk

GTK+ (formerly GIMP Toolkit, sometimes incorrectly termed the GNOME Toolkit) is a cross-platform widget toolkit for creating graphical user interfaces. It is licensed under the terms of the LGPL, allowing both free and proprietary software to use it. It is one of the most popular toolkits for the Wayland and X11 windowing systems, along with Qt.

gtkmm (formerly known as gtk-- or gtk minus minus) is the official C++ interface for the popular GUI library GTK+. gtkmm is free software distributed under the GNU Lesser General Public License (LGPL).