I think what you're saying is that you use a gmail address for sending and bounces, right?
This means your delivery server is gmail. Did you try to also use it as a bounce server using imap/pop3 ?
Mailwizz looks for unread messages when it connects to imap/pop3 as a bounce server, so if you still have the emails in that inbox, mailwizz will be able to fetch them.
Another way to go about this is to send your emails through amazon ses / mailgun / sendgrid / etc where mailwizz will do bounce processing automatically for you.