This plugin enables the use of Behavior Driven Development to test your Grails Application,
The library behind this is easyb, easyb is a BDD library written in groovy.
For example, you can test your app like this:

given "an invalid zip code", {
  invalidzipcode = "221o1"
}
and "given the zipcodevalidator is initialized", {
  zipvalidate = new ZipCodeValidator()
}
when "validate is invoked with the invalid zip code", {
  value = zipvalidate.validate(invalidzipcode)
}
then "the validator instance should return false", {
  value.shouldBe false
}


You can find more information about this plugin and latest released in the following URL: 
http://www.urubatan.info/tag/easyb-test/