It took me a few moments to find the correct SMTP setting of using Gmail server.
Hope this post can save time for those who want to use gmail smtp service like me.

1. Set the SMTP setting in your forum as the picture below
forum SMTP setting

 

2. Log in the gmail account you want to use for sending email and enable "Allow less secure apps" in the last part of  Sign-in & security page of account setting

Gmail account setting