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 still can’t recover you mails, these links will provide further help