In some IDE (like IDEA) graphic representation of your Grails domain classes is built-in. But if you use another IDE, eg. SpringSource Tool Suite (like me), you can supply this lack with the Class Diagram plugin.

Installation is really simple: first, check that in your system is installed Graphviz (under Debian/Ubuntu

sudo apt-get install graphviz

) than into your app folder type

grails install-plugin class-diagram

Re-run your app and at http://localhost:8080/<your_app>/classDiagram you can see the class diagram generated. The result could look like the following: