A GUI is composed of elements that in some natural way inherit from each other. E.g. there are push buttons, radio buttons, which are all buttons. And buttons and many other elements are widgets. The main windows, dialogs and so on are frames or windows.
Motif already had an object oriented design, implemented using standard C. They even had public and private data members, faked using two header files defining two structures for every element, one of them for Motif itself, including the private members, the other one with just the public part, layout compatible. They've got virtual functions, implemented with function pointer arrays, and so on. And I can see the same thing in GTK.