To set an automated out of office reply follow these steps
Go to Gmail Settings (top right gear icon) > click "See all settings"
Select the “General” tab
Scroll to "Vacation responder" (very bottom of the page)
Select the button that says “Vacation responder on”
Set the start and end dates and write the message you want people to see
Click Save changes
This will result in an automated message getting sent to anyone who sends you an email during the start and end dates!