There are a few GUI libs/modules for Python (which of course are cross-platform) but it seems that ST does not ship any of them. At least I could not find anything. While I understand not including any of them to reduce the size and complexity of the Python package I think that the possibilty for GUIs in Python would open another wide varity of plugins to be written. It could be used to create a GUI for ST's settings for example. You (@Jon) could also provide an API for your own Framework you use with ST - I don't know if that'd be worth it, that's your part.
Please leave a statement which explains why you do not want to implement the ability to create GUIs in plugins or note your thoughts about this.
If you are a package developer, post whether you want a GUI framework in Python plugins or not (though, I don't think there will be many people against this).