Mark all Unread mails in gmail as Read

1. Go to your Settings/Filters page and create a new filter.
2. In the Has the words field enter “is:unread” (without quotes), and click the Next Step button.
3. You’ll get a message warning you that this type of filter won’t be applied to new mail, but that’s OK.  Click OK to continue.
4. Check the boxes next to Mark as read and Also apply filter to … conversations below.
5. Click Create Filter button, and you’re done.  You should delete the filter once you finish since it won’t be needed anymore.