There is a grails plugin :
http://www.grails.org/plugin/create-domain-uml .
Install it via the following command :
grails install-plugin create-domain-uml
To run the plugin:
grails create-domain-uml
This will create a file DomainUML.html in your project root which contains the diagrams.
you can see also : http://www.svenlange.co.za/2010/01/generating-uml-class-diagrams-from-grails-domain-model/