As per GWT 2.7.0 (RC1) Release Note, GWT Designer is deprecated. I use the tool for some complex UI design in my project. Just don't know why this happens and what are alternatives without it.
The GWT Designer is no longer the focus of the GWT Steering Committee (Members include Google, Vaadin, Sencha etc )
As stated the code is open sourced and if anybody is interested they can take ownership of it to revive it. Going back in history Google bought the company that built the designer and utilised some parts of it initially and later on abandoned the designer with very little commits over last few years. So its been in coma for more than couple of years now with few changes added to support yearly eclipse releases.
Looking into the source -> https://github.com/gwt-plugins/gwt-designer
They are adding support for next version of eclipse too.
Of course Vaadin and Sencha have their own take on the Designer. Their focus might not be purely native GWT. Reference: