Welcome to the Atlassian Community!
This means that the machine you are running Jira on is not able to resolve smtp.gmail.com as a valid server address. The most likely problem is that its DNS client is not configured correctly.
Try logging into the server, and, on the command line, running nslookup smtp.gmail.com to get a bit more about the underlying error.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.