well GBUI is a library built to work with JME which uses a system similar to CSS to control the entire layout of the GUI.
Might want to check that out.
GBUI Doesnt do that either, its a typical layout driven gui. Like Swing. Im not sure there is really anything like you descriped current built. However writing a file parser in say XML to create components would not be all that hard. Or the method i would use, would be to use GBUI with stylesheets and then for the component creation use a scripting language and engine. Java comes with loads, and non are really complicated.
GBUI doesn't support layout design by xml nor some other files, that's correct. It only supports changing the design by BSS files insomuch that you can change the graphic or colors of things from there.
I agree with @d0md0md0m that parsing some xml or other file to implement a layout wouldn't be difficult - it would be a matter of defining a markup language, or even easier, taking a stripped down html (like html 2 or something) and implementing a parser for it.
I currently have a w3c parser for html and css that I'm working to implement. The idea is that the parser is up to date with the latest HTML and CSS versions, however, not all of the attributes would be available in GBUI. Then parsing an HTML layout and/or CSS styles and providing that layout in either a class file that represents the layout or on the fly (preferred).
There are a lot of easy things that can be done to improve the ease of layout by separating the layout from the developers to the designers, I'd just need to discuss exactly what you're looking for then come up with a feature set and implementation design doc so that we don't just haphazardly dump them in.