You need to login to google and change the security settings so less secure apps can obtain permission to use services etc.
"account, security, scroll down to the bottom and turn off less secure app access", should cure it
Once done my pi3's are set to
smtp.gmail.com
465
SSL
2 Jan 2019 10:35:51 Email alert sent: "Alert: Test alert"
I wouldn't advise doing this and it's absolutely unnecessary for DslStats.
It used to be needed but since Eric added StartTLS I've disabled this insecure setting on gmail and DslStats works perfect.
I'd recommend setting a 3rd party password for DslStats (through Google) before lowering security across the board if you cannot get it to work.