If I have created a domain class in Grails that has a data field of a custom data type, does that data type also need to be a domain class in order to be saved in the database?
Grails understands how to persist strings, numbers, dates, booleans, etc, as well as other domain classes. If you want to persist a type that isn't directly supported you can create a custom Hibernate UserType to tell Hibernate how to store and retrieve your data. See the docs here: http://grails.org/doc/latest/guide/GORM.html#customHibernateTypes