I've not had much luck with smtp-relay.gmail.com so I just skip the relay and use smtp.gmail.com, port 587 with STARTTLS and have no issues. The relay server needs IP address authentication and has more antispam filters to get in the way.
If you must use the relay service, you will likely need to do a Wireshark trace to find out what Gmail is sending back to the printer that it can't interpret. It could be as simple as needing less secure apps to be enabled, or to disable 2 factor authentication if it is in use.
There is a very likely chance your firmware just isn't new enough (It's from Nov 2014) to handle the required SHA-2 for gmail. If that is the case, then a simple update to current firmware should be sufficient. And it will enable the device to do SMB scan to Windows 10 and Server 2012, so you will probably need it soon anyway. So grab the current version here, which is as of this posting, 061.121.227.03404