How to set up an auto-reply or an out-of -office reply on gmail?

To set up an auto-reply or an out-of-office reply on gmail.

First login to your account using web interface i.e., www.gmail.com
Go to settings by clicking on the gear icon on the top-right corner as shown in the below picture.

In General Tab, scroll down till you find Vacation Responder as shown below.

Then select Vacation Responder on and fill in the First day, Last day, Subject and Message fields as per your requirement.
After completing the all the details, click on Save Changes.
Once the changes are saved, you will be good to go.