This is to help anyone else who comes across this problem - I’ve had a problem for a couple of years now and finally got it working for me again. LO version Version: 184.108.40.206 (x64), Windows 10 Home.
First make sure access for less secure apps is turned ON in gmail. Even if you have turned it on before, it turns itself OFF every so often so check it is still on before you test/send. (https://myaccount.google.com/lesssecureapps)
I have both POP and IMAP enabled in gmail anyway -so not sure which you need - think it is IMAP (gmail; settings;forwarding and POP/IMAP)
Open a writer doc, then Tools, Options, LO writer, Mail merge emails
Use the exact name you use on your Gmail account
This I think is the important bit
Email address - it is important to use @googlemail not @gmail if that the address associated with your google account.
Then SSL ticked, port 465
Server authentication - the outgoing mail server requires separate authentication, user name is full google mail address (again @googlemail if that is what your address is) and gmail password.
And hopefully that will work for you too!
Be aware that if you haven’t said the server needs authentication and try ‘test settings’ it will seem to show no problems, but your emails won’t send.
You need Test settings to show successful network and outgoing server connections after saying server authentication is required.