No application but you will be able to reply to the GMail notifications.
That's not completely true as I can reply to my emails from the watch using GNE app that unfortunately is not anymore available in the store... That is a great app for notifications...So, on the watch, I can read my Gmail e-mails, but if I want to reply, I have to do it through the e-mail app on the phone (populated by the native e-mail app on my phone)?