The best way to auto respond for an email is to use a filter.
- In the top right, click Settings
- On the left side, click Filters.
- Near the bottom left, click the plus(+).
There are many options, but for a general auto-responder, select "To" and enter the email address that is responding. This should catch all incoming email and respond.
- Select Out of office message and enter a subject and any additional information in the text.
- Click Save.