Heres an example of my actual migration than ran and worked:
changeSet(author: 'me', id: 'createCodeTrees') {
grailsChange {
change {
new CodeTree(name: 'BAN8', enabled: true, defaultTree: true).save(failOnError: true)
new CodeTree(name: 'BAN8OLD', enabled: true, defaultTree: false).save(failOnError: true)
new CodeTree(name: 'BANPPRD', enabled: true, defaultTree: false).save(failOnError: true)
new CodeTree(name: 'BANADVM', enabled: true, defaultTree: false).save(failOnError: true)
new CodeTree(name: 'BANSTFA', enabled: true, defaultTree: false).save(failOnError: true)
new CodeTree(name: 'BANHRPY', enabled: true, defaultTree: false).save(flush: true, failOnError: true)
}
}