Can GPT reply to my email when I'm not at my desk?

No, automated replies are not enabled for emails as of now. Auto replies are only enabled for messages coming to Chatbot. While automated email replies are possible, generative AI capabilities are not advanced to be relied upon blindly. Human supervision is still required. Also, to respond to the email, the AI models will need information that might not be available publicly.  

What you can do, though, is click the Calculus icon while writing an email. In the prompt window, describe the email you want GPT to draft, and GPT will give you a draft for your email.