Gtk.AboutDialog:license
"license" String : Read / Write
The license of the program. This string is displayed in a text view in a secondary dialog, therefore it is fine to use a long multi-paragraph text. Note that the text is only wrapped in the text view if the "wrap-license" property is set to true; otherwise the text itself must contain the intended linebreaks. When setting this property to a non-null value, the Gtk.license-type property is set to Gtk.License.custom as a side effect.
Since 2.6