I found this thread while looking up the best configuration to use a Gmail account for email delivery and thought I'd add an update to the OP.
I used all of the settings mentioned in my configuration and emails would not work. I found that Gmail have introduced a security setting to prevent less secure access, which this is classed as.
To get things working you need to log in to the Gmail account you're using and from this page: https://myaccount.google.com/security
change "Allow less secure apps" to ON.
If like me your email account is managed through G-Suite then you'll first have to allow the user to do this from the Admin Centre Security Settings. "Allow users to manage their access to less secure apps".
Once I'd done this the "Send test email" was delivered to my board admin account and new registration emails were successfully received.