Here you go
Horizontal Panel will create an table and if you gave the width on the panel with in that width widget will share the equal space .
So
Set the display type block
so they will arrange in side by side in the whole width
HorizontalPanel panel = new HorizontalPanel();
panel.setWidth("70%");
panel.getElement().getStyle().setDisplay(Display.BLOCK);
panel.add(new Label("Lable"));
panel.add(listbox);
RootPanel.get().add(panel);