The feature is for both Gmail and Hangouts Chat, with a banner appearing at the bottom of the email compose panel or the chat window to warn senders that the recipient has an out of office entry on their calendar. The banner also says when the recipient is due back, which is handy.
The notifications are only available for users of G Suite, Google's business tool suite. That's okay though, as it's likely that business users would be the only ones using out of office entries anyway.
The feature will be enabled in G Suite automatically and will be rolled out over the next few weeks.