I think the reason you can't communiate off of port 587 is because of Go, not gmail. The go doc for the PlainAuth function says:
func PlainAuth(identity, username, password, host string) Auth
...
PlainAuth will only send the credentials if the connection is using TLS or
is connected to localhost. Otherwise authentication will fail with an error,
without sending the credentials.
587 is the standard outgoing port for TLS communications via SMTP.