Ok I figured this out.
In the Grails 2.3.7 release notes it says to upgrade your hibernate version. I failed to do that.
http://grails.org/2.3.7+Release+Notes
runtime ':hibernate:3.6.10.10'
Then after I did that I stopped getting that error and got another error
java.lang.ClassNotFoundException: null
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
at ....MigrationUtils.createInstance(MigrationUtils.groovy:220)
at ....MigrationUtils.getDatabase(MigrationUtils.groovy:57)
at ....MigrationUtils.getDatabase(MigrationUtils.groovy:116)
at DbmGenerateGormChangelog$_...doCall(DbmGenerateGormChangelog:52)
at ....MigrationUtils.executeInSession(MigrationUtils.groovy:132)
at DbmGenerateGormChangelog$_run_closure2.doCall(DbmGenerateGormChangelog:51)
at DbmGenerateGormChangelog$_run_closure1.doCall(DbmGenerateGormChangelog:33)
I've never had to specify the hibernate dialect for oracle before, but apparently in the most recent version of the migrations plugin (1.3.8 currently), you have to.
dialect = "org.hibernate.dialect.Oracle10gDialect"
I was already specifying the dialect for MySQL so I don't know if it would produce the same error if removed.