GTK+ (GIMP Toolkit) is a cross-platform widget toolkit for creating graphical user interface. Developed by GNOME Foundation
License: Open Source
GTK+ is primarily designed for Linux and GNOME applications, making it a lightweight option for simple applications, while Qt is a more robust, cross-platform toolkit suitable for complex UIs with extensive features and commercial support. Qt's performance and tools for UI design make it a preferred choice for larger, more intricate applications.