When authenticating to smtp.gmail.com you have to do that with your username and password. I want the reply to to be a alias orders@mystore.com how can I add a alias reply to address.
I'm not sure I follow, you want to use a different email address from the one you have entered in the main admin settings of the control panel - is that right?
You can have extra email addresses by separating them by a comma, or you can set a rule in your email program to forward mails with a specific subject line I believe but I can't see a way of defining a different email address.