1. added GWTx-20070110.jar to my eclipse project's cp (gwt-1.3.3)
2. Declared inheritance ( <inherits name='com.googlecode.gwtx.Java'/>)
3. Using Property support yields:

[ERROR] Errors in 'jar:file:/C:/[...]/lib/GWTx-20070110.jar!/com/googlecode/gwtx/java/io/emul/java/io/ByteArrayOutputStream.java' -> [ERROR] Line 81: The method close() is undefined for the type OutputStream -> [ERROR] Line 256: The method write(byte[], int, int) is undefined for the type OutputStream

Any hints /ideas ?

Reorder your classpath: http://code.google.com/p/gwtx/wiki/FAQ