To start with the confusion: Gtk is a GIMP toolkit , and all the different things you mention in the title of the question are related, but not exactly the same. PyGTK is the link between Python and GTK, and PyGObject is the preferred link if you go this route.
Here's a nice brief overview of all the different GUIs for Python development, which should briefly explain these terms for you to get an overview. And, as gauden suggested in the comments, wxPython is a very common way for cross-platform applications.
For now, you should at least consider Kivy , itβs fun to have a program, and it even gives you options for expanding the cross-platform bit for mobile devices. It supports transparency ( or opacity in Kivy terms )
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