GTK3 is a widget toolkit used for creating graphical user interfaces, formally known as the GIMP toolkit. The draft Java Enhancement Proposal titled, Modernize the GTK3 Look and Feel implementation, calls for the rewriting of the current GTK2 Look and Feel so that it uses GTK3 instead.
GTK3 implementation will not replace GTK2. It is important to note that one or the other, not both of these can be used at runtime.
You can access the GTK3 reference manual at https://developer.gnome.org/gtk3/stable/.
