You must know the RadioButton or you wouldn't have got this far in I.T. It all works the way you expect with the exception that you'll definitely want a RadioBar - a set of RadioButtons where you can get the selected index, the selected text and the selected value. There isn't one in GWT. Perhaps I'll write one.
VerticalPanel panel = new VerticalPanel();
panel.addStyleName("demo-panel-padded");
panel.add(new RadioButton("group", "One"));
panel.add(new RadioButton("group", "Two"));
panel.add(new RadioButton("group", "Three"));