Google explained from 0.08% of Gmail users affected by the bug. And Google Apps Status Dashboard explained that the Google engineers are working to restore full access to the affected user.
Google writes,''For Gmail users, with reference to the missing message in Gmail, our technicians will restore it immediately.''
Free application for Mac, PC and Linux to backup data called Gmail Backup. The system will download the 2.4% user email and do not require much time to backup data.