As part of amazing goodies of grails is the ability to extend your applications - registering new classes programmaticaly is a breeze
As the grails guru Burt points out here it just requires minimal effort;
//first create a bean definition
def myBeanDef = new GenericBeanDefinition()
//set bean class
myBeanDef.setBeanClass(YourBeanClass)
//set scope
myBeanDef.setScope(BeanDefinition.SCOPE_SINGLETON)
//set others
//register
grailsApplication.mainContext.registerBeanDefinition "someName", myBeanDef
Programming in grails is really a pleasure.