This is up to the authors, so you will have to ask them once you are done with your widget. As a general guideline, widgets that are generally useful, work, and are not a disgrace to the widget set will gladly be included.
The new widgets that get added to GTK+ are generally either replacements for existing widgets that are no longer deemed to be adequate, or have been developed externally to GTK+ but have been widely tested. One possible avenue for this is via libegg.
Before you spend months of your valuable time implementing your revolutionary widget, it is highly recommended that you get some feedback on your idea via the appropriate mailing list.