Now you are perhaps stepping on controversial ground as web designers and Swing developers meet on the web page writing the code in Java
GWT comes with a series of widgets wrapping standard HTML component (Labels, Buttons, Form elements etc) and a number of panels (HorizontalPanel, VerticalPanel, DockPanel, DisclosurePanel etc).
I think Swing users will be fairly familiar with the panels provided. There are perhaps some subtle name differences, but on the whole the concepts are there. Where you might get caught out is how some of the panels work - because of the constraints of the browsers GWT supports.
A good example of this is the FlowPanel which flows elements as the browser dictates. You might expect if you put several Labels in a FlowPanel they would run from left to right; however, they appear under each other since this is the common denominator way for all supported browsers to do this. Early versions of GWT supported a strict Flow approach, but that was not consistent across all browsers; so to keep consistency the lowest common denominator is now given.
In "the other corner" so to speak would be web developers that would prefer to position and style everything using Cascading Style Sheets as opposed to layout managers.
My honest view is that there is room for both approaches in your GWT application. Where you as a programmer need/prefer to rely on a structure being in place then use the layout managers; where you don't then use the default CSS style classes that are provided for all widgets and let the web designers perform their magic.
It does mean as a Java programmer you have to get an understanding of style sheets etc, but it gives a great amount of flexibility and freedom if you need to reuse components / change the style quickly.
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