Try like this
You have to add the widgets after you set the align center
VerticalPanel vpnPanel = new VerticalPanel();
vpnPanel.add(scrollPanel); //all controls added to scrill panel with help of set widget
vpnPanel.setHorizontalAlignment(HasHorizontalAlignment.ALIGN_CENTER);
HorizontalPanel btnLayoutPanel = new HorizontalPanel();
btnLayoutPanel.setSpacing(10);
btnLayoutPanel.add(closeButton);
vpnPanel.add(btnLayoutPanel);
decPopuppanel.setWidget(vpnPanel);