Assuming you have aliases setup in Gmail, it’s fairly easy to configure the Mail application on OS X to respond to the emails using the appropriate alias.
If you haven’t already, configure your Mail app to your gmail account.
Finally, to configure your aliases, go back into your account settings in the Mail app and comma separate each alias under the Email Address: field. It’s far from obvious but a tooltip will appear after a moment which reads:
Enter one or more email addresses, separated by comma.
You may also find it useful to (configure your iOS)[/apps/ios/mail/configure-ios-mail-to-respond-with-gmail-aliases/) device the same way.