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 app in the Development category. There are more than 25 alternatives to GTK+ for a variety of platforms, including Windows, Linux, Mac, Android and iPhone. The best alternative is Qt, which is both free and Open Source. Other great apps like GTK+ are Electron (Free, Open Source), Flutter (Free, Open Source), wxWidgets (Free, Open Source) and Avalonia (Free, Open Source).
Qt is a cross-platform application framework that is widely used for developing application software that can be run on various software and hardware platforms with little or no change in the underlying codebase, while having the power and speed of native applications.
Qt vs GTK+ opinions
Most users think Qt is a great alternative to GTK+.
Se7enTime GTK + and QT are the two major players that dominate the Desktop Environment on GNU / Linux today.
wxWidgets is a C++ library that lets developers create applications for Windows, OS X, Linux and UNIX on 32-bit and 64-bit architectures as well as several mobile platforms including Windows Mobile, iPhone SDK and embedded GTK+.
Avalonia is a WPF-inspired cross-platform XAML-based UI framework providing a flexible styling system and supporting a wide range of OSs: Windows (.NET Framework, .NET Core), Linux (GTK), MacOS, Android and iOS.
Uno is the platform for building native mobile, desktop and WebAssembly apps with C#, XAML from single codebase. It's a Universal Windows Platform Bridge that allows UWP-based code (C# and XAML) to run on iOS, Android, and WebAssembly.
The PySide project provides LGPL-licensed Python bindings for the Qt cross-platform application and UI framework. PySide Qt bindings allow both free open source and proprietary software development and ultimately aim to support all of the platforms as Qt itself.
Dear ImGui is a bloat-free graphical user interface library for C++. It outputs optimized vertex buffers that you can render anytime in your 3D-pipeline enabled application. It is fast, portable, renderer agnostic and self-contained (no external dependencies).
Dear ImGui vs GTK+ opinions
pros, cons and recent comments
Is the most popular GUI library in Github for C++ is even more popular than GTK and QT and is full open source unlike QT, and has better look than QT and GTK