The update Gmail app for Android, lets you send auto reply as vacation response right from your Android phone or tablet. To setup the auto reply in Gmail app, do as follows:
1. Open Gmail on your Android device (Make sure it is updated to the latest version: Update Gmail from Google Play)
2. Tap the 3dot icon and go to Settings.
3. Select the email address you want to set the auto reply option
4. Tap Vacation Responder.
5. Set the start and end times, as well as the Subject and Message
6. Mark "Send only to my contacts" if necessary.
7. Press Done and you are all set