Peter Ledbrook wrote an excellent introduction article about Grails ORM a.k.a. GORM in which the importance of Hibernate and the usage of sessions are highlighted. A quote:

Hibernate is a session-based ORM framework.

A session is retrieved from Hibernate’s SessionFactory (org.hibernate.SessionFactory) which implements a design pattern that ensures that only one instance of the session is used per thread. GORM uses this factory to get the session – and so should you if you ever need the true power of executing raw SQL in your Grails application!

An example of executing SQL yourself:

 	String sql = "some update SQL"
	Session session = sessionFactory.openSession()
 	Connection c = session.connection()
	try {
		try {
			Statement s = c.createStatement()
			s.executeUpdate sql
		}
		catch (all e) {
			log.warn "Error executing statement $sql ($e)"
		}
	}
	finally {
		session.close()
		c.close()
	}

Essential is to have the session factory auto-injected into your class, by putting the following somewhere on top:

    def sessionFactory