I've attached a proposed patch which mainly solves this issue.
There is one limitation, though. I didn't manage to make groovydoc's link method to work (meaning, links to the external apis doesn't exist). Whatever I tried, ended up with an exception. I'm not sure, but it seems there is a bug in Gradle's org.gradle.api.tasks.javadoc.AntGroovydoc.groovy while trying to execute org.gradle.api.tasks.javadoc.Groovydoc.java. Maybe someone can take a look at it.
Although external links are missing, I believe that having api docs for all Grails classes (groovy and java) will be an improvement.