The ignorance in this thread is rather concerning.
Let me correct: GTK+ is not C++, it's C. Qt is very cross-platform: it supports about 10 platforms, including Linux, Windows, and macOS. You are not doing anything close to compositing raw pixels in Qt or GTK+. They have layout managers that are specifically made for typical application UIs, and they have lots of ready-made widgets. Qt also has QML, which is a declarative UI descriptions language kind of like HTML/CSS but much more productive because (gasp) it's actually designed from the ground up for applications, not websites.
Let me correct: GTK+ is not C++, it's C. Qt is very cross-platform: it supports about 10 platforms, including Linux, Windows, and macOS. You are not doing anything close to compositing raw pixels in Qt or GTK+. They have layout managers that are specifically made for typical application UIs, and they have lots of ready-made widgets. Qt also has QML, which is a declarative UI descriptions language kind of like HTML/CSS but much more productive because (gasp) it's actually designed from the ground up for applications, not websites.