All references have now been changed from GMT to UTC. In addition the definition of the JMSExpiration message header field has been clarified.
In the JMS 1.1 specification, section 3.4.9 "JMSExpiration", a message's expiration time was defined as "the sum of the time-to-live value specified on the send method and the current GMT value".
However the JMSExpiration header field is a long value and the specification does not define how the expiration time is converted to a long.
This has now been clarified to state that it is "the difference, measured in milliseconds, between the expiration time and midnight, January 1, 1970 UTC." This definition is chosen to be consistent with the java.lang.System method currentTimeMillis.
The updated text can be seen in the JMS 2.0 public draft, section 3.4.9 "JMSExpiration" and in the API docs for the Message method getJMSExpiration.