The principle is that behind the curve there is a mathematical function that will render each point of the curve. Gimp cannot do this. Perhaps the excellent pyQt and pyKDE modules could also be used. If your interested, that is. This part is more or less complete already.
Different languages are just different ways of doing the same thing.
That depends on the toolkits in question, of course. In the particular case of switching from Tk to GTK, there are several advantages. You have to implement tooltips yourself, for instance, unless you want to rely on an extension. OTOH, Tk is better documented and more stable. Tkinter, the Python interface to Tk, uses tcl to communicate with Tk, so I actually have two interpreters in Sketch, Python and tcl.