If you are choosing a Gui library, Qt will be great.
For a beginner , gtk is easier to learn than qt.
But after you learn how to implement signal and slots, and subclass a QObject, everything will be easier with qt.
When i started to learn gtk, i tried to create a simple drawing app, and failed to draw over a pixmap using cairo.
But then i switched to qt, and everything became easier with QPainter.
Also, there are some convenient classes, which helps a lot.
Till now, i have made a browser, a pdf viewer, a paint app, a printing app, raspberry pi configuration editor, calculator, markdown editor, image files to pdf converter using either c++ qt or PyQt.
My Qt projects at ... https://github.com/ksharindam