Mapping your domain classes via XML is pretty straightforward. Simply create a hibernate.cfg.xml file in your project's grails-app/conf/hibernate directory, either manually or via the create-hibernate-cfg-xml command, that contains the following: 
<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!-- Example mapping file inclusion -->
        <mapping resource="org.example.Book.hbm.xml"/>
        …
    </session-factory>
</hibernate-configuration>

The individual mapping files, like 'org.example.Book.hbm.xml' in the above example, also go into the grails-app/conf/hibernate directory. To find out how to map domain classes via XML, check out the Hibernate manual.

If the default location of the hibernate.cfg.xml file doesn't suit you, you can change it by specifying an alternative location in grails-app/conf/DataSource.groovy:

hibernate {
    config.location = "file:/path/to/my/hibernate.cfg.xml"
}

or even a list of locations:

hibernate {
    config.location = ["file:/path/to/one/hibernate.cfg.xml","file:/path/to/two/hibernate.cfg.xml"]
}

Grails also allows you to write your domain model in Java or re-use an existing one that already has Hibernate mapping files. Simply place the mapping files into grails-app/conf/hibernate and either put the Java files in src/java or (if the domain model is stored in a JAR) the packaged classes into the project's lib directory. You still need the hibernate.cfg.xml though!