There have been several projects over the years that do scaffolding (although none that I know of that use ValidateThis). They are mostly tied to a specific framework, I believe that CFWheels, ModelGlue, ColdBox and Fusebox have all done this or had support via plugins / extensions. CFWheels is based on Ruby on Rails so would be the closest fit to Grails.
There are two independent projects I can think of that are not tied to the framework which may be worth looking at as they generate code so could be adapted to your needs:
A few other projects show up on RIAForge but I don't know much about them: