<pre>frame (title:'Demo') {
    menuBar {
        menu('File') { 
            menuItem 'New' 
            menuItem 'Open'
        } 
    } 
    panel {
        label 'Label 1' 
        slider() 
        comboBox(items:['one','two','three'])
    } 
}  
frame.show()
</pre>
http://blogs.sun.com/geertjan/entry/how_cool_is_swing_in

Example of using swing with groovy and CSS - http://www.jroller.com/aalmiray/entry/css_swing_groovy_you_bet