Hi Brad,
That is my understanding too, that it is CA licensing Java in this case and not yourselves when you use the CA API Gateway appliance. If you use the software form factor rather than our virtual or hardware appliance images, however, then I believe the Java part does become an onus for the customer in that case. As long as you're using our provided images, you won't need to worry about licensing Java with Oracle on them.
To answer the original question, no - we do not currently support OpenJDK.