There aren't any plans to notify when it comes back up. I don't recall 100% why that was, but I think it's because there are times when it's assumed to be up or when it comes back that would cause a lot of false positive and/or redundant e-mail notifications.
I have posted a Feature Request for this bug. It appears that if a Gateway goes down, comes back up, and goes down a second time, the second email is not sent, IF the contents of the last notification sent is the same. Saving the current state of the GW or sending an email upon GW up status would solve this bug.