Try using this:
Code:
var invalidSession = function(){ window.location = '<%= request.getContextPath() %>/login.jsp'; };
Ext.lib.Ajax.on('status:403', invalidSession );
For Spring Security with GWT, you would have to code the FormPanel to perform a traditional form POST as opposed to an XHR call. The issue with doing it through an XHR call is that the return data is not in the proper format so GWT does not know what to do, or if you are stuck on using XHR, then you would have to write your own RequestCallback as a handler and parse the return information manually for the onSuccess and onFailure.
Good luck.