Before doing anything, make sure that you enable IMAP access to your Gmail account. Click here for information on how to do that.
In addition to enabling IMAP, make sure that the "All Mail" or "INBOX" labels are set up to be synced over IMAP!
Getting the app to speak requires that you first click on the "Accounts" button to configure your accounts.
You should now see a list of accounts. Click on any of them to configure that particular account.
Once you click on the account, you will be prompt to allow us to contact Gmail with OAuth2. Once you accept, you will be directed back to your account list.
That is all there is to receiving Gmail notifications!