UI je vecou toolkitu, takze designer je samostatny nastroj nezavisly od jazyka. gtk ma glade, mozes si ulozit xml a nacitat to z programu (python, ruby, perl, C, ...) a vie to napojit automaticky signaly na metody podla toho co specifikujes. pre wxpython by mal byt zase wxglade. v qt asi potom pouzijes qt designer.
https://python-gtk-3-tutorial.readthedocs.io/en/latest/builder.html
http://wxglade.sourceforge.net/index.php
https://nikolak.com/pyqt-qt-designer-getting-started/