GWT modules are defined in special XML format. Developer of a new module has to maintain such file as well as module's java source code. Documentation of the GWT project explains each option which can be put in module descriptor, however one thing is missing there - reference to XML Schema or DTD.
Appropriate DTD has been submitted some time ago, and you can find it here. In order to use it in your own modules just add this DOCTYPE to your module xml file :
<!DOCTYPE module SYSTEM "http://google-web-toolkit.googlecode.com/svn/releases/1.5/distro-source/core/src/gwt-module.dtd">