If Gmail has not deleted the messages from the server, you can recover deleted emails after deleting them from your trash folder. You must open your Gmail then click the drop down grey arrow on the left from your search box at the top of the screen. It will then ask you several question such as to or from, subject, key words or date within. Click on the blue search button on the bottom left of the search box and all of your deleted emails should be revealed.
If you delete an email, it stays in your Trash for 30 days. After 30 days, emails are permanently deleted from the trash and the server.
If you still can’t recover your mails, these links will provide further help