GORM is Grails' object relational mapping (ORM) implementation. Under the hood it uses Hibernate 3 (an extremely popular and flexible open source ORM solution) but because of the dynamic nature of Groovy, the fact that it supports both static and dynamic typing, and the convention of Grails there is less configuration involved in creating Grails domain classes.
GRAG1.0 is a free domain object generator for the Grails framework (http://grails.org).
You can use The GRails Application Generator (GRAG) to get started with Grails using an existing database. It will generate Grails domain objects including constraints and relations. After generating the domain objects, you can continue using Grails with all it's power.