Is there any message loss in the GMS disaster recovery process?
No. Part of the disaster recovery process is backing up all files related to users email messages. Provided this is done properly, there is absolutely no loss of emails received or in the process of being sent. Any incoming message that had not completed will be held on the remote system, waiting to retry delivery once it has been brought back up.
GMS has been praised in the past specifically for its 0% message loss and so as an administrator of a GMS system, you can be confident that no important messages will be lost.