Turning on auto response for GMail is easy. Simply follow these instructions.
From your GMail account click the settings wheel on the upper right and then Settings:
Under the General tab (default) scroll down to the bottom where you will see Vacation Responder:
In the message box type in the message you'd like people to receive when they send you an email. Be sure to click Vacation Responder On and, also, put in the dates you will be unavailable. If you do not put a Last Day and a check mark next to Last Day it will continue to send the auto response until you turn off Vacation Responder.
Be sure to click the Save Changes button on the very bottom.