Try,
String value = com.google.gwt.user.client.Window.Location.getParameter("orderId");
// parse the value to int
P.S. GWT can invoke native javascript which means if javascript can do the stuff, GWT can do it too; e.g. in GWT, you can write
public static native void alert(String msg)
/*-{
$wnd.alert("Hey I am javascript");
}-*/;
In this case, you can even use existing javascript lib to extract param’s value in the querystring.