There is a simply splendid 4-page(!) article written by Jason Rudolph taking you step-by-step with screenshots and all to get a scaffolded Grails application working with a legacy MySQL database using Hibernate XML mapping.
This really demonstrates the power that Grails has to offer in terms of being simple on the surface, but having all the power of Hibernate underneath. Note that Jason could equally have written his domain model and Java and used annotations to achieve the same effect. See the page on Grails' Hibernate integration in the user docs for more info.