Thank you for your response, but I think it missed the point.
As I said, the problem with the custom parser is solved, thanks to the custom parser from here: http://code.google.com/p/gxt-uibinder/
. I can use all the Widgets (that I need) from GXT without a problem. So it looks to me, what needs to be done is, that GWT-Designer needs to recognize the container it droped into the layout area as container and allow to drop Widgets into it.
The actual problem is again, that GWT-Designer allows me to open a UiBinder file, and offers me the posibility to drop a layoutcontainer from GXT in the layout area. But then, it does not recognize that this is a layoutcontainer so it does not allow me to drop stuff into it. Sorry, but to me this looks, walks and sounds like a bug. Especially since all the visual GXT Widgets can be used outside of GXT containers. So the general functionality is available - it is even possible to drop ColumConfig's into a Grid.
When I am not mistaken, GWT-Designer is, or will be available as source. So you could even point me to the file that needs to change to make this happen. But your blocking attitude is very unfortunate.
And, when you so absolutely and definitely don't support this, than it would be more helpful to remove the Widgets you don't support in UiBinder from the Panel, so we don't need to be hit by wondering why it does not work.