Well I hope this helps you and so you can call Carnival and get this straightened out.
If you bought the Gift Certificates in the State of California, it is illegal for gift certificates to expire per the laws of the State of CA.
You need to call Carnival and tell them that you want to use your GC. At first if they tell you no ask to speak to a Supervisor and ask again. If again you are told no tell them that it is illegal in CA where you purchased the GC and and inform them of this law and to put you on hold while they verify this info with their legal dept and that you would like to hold. Use it for another cruise if you arent able to use it for onboard credit.
Hope this info helps. If anyone else has this issue please check what the law states for your State of Residency.