It has happened since Gmail exists and with the iPhone since it was released in 2007.
Nothing extraordinary, unless you see it constantly, maybe the configuration is not right, or you see it while connected to another ISP, or Wi-Fi network, 3G?
As I recall I was seeing that frequently and was because in my Mac's Mail App the outgoing smtp server was automatically configured for a particular ISP, and I synced the iPhone via iTunes to obtain the email. It was setup as POP mail, and when trying to connect to Gmail servers there were those errors.
The way I got rid of them was to setup the Gmail accounts as IMAP and not syncing via iTunes.
Check in Google's Gmail help documents if you need to re-configure them to avoid errors.