GTK is described as 'Multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK is suitable for projects ranging from small one-off tools to complete application suites' and is a popular software framework in the development category. There are more than 25 alternatives to GTK for a variety of platforms, including Windows, Linux, Mac, Android and Self-Hosted apps. The best GTK alternative is Qt, which is both free and Open Source. Other great apps like GTK are Flutter, Electron, PyQt and Photino.
GTK + and QT are the two major players that dominate the Desktop Environment on GNU / Linux today.