Hello,

it depends which port you want to use for GMAIL SMTP
more on gmail setup here:
https://support.google.com/mail/answer/7126229?visit_id=1-636531230957604814-933739698&hl=en&rd=1

If you want to use port 465, than:
Port 465 with SSL ticked (use the checkbox in Email and Conferences module >> Settings >> Email SMTP Server >> SMTPS (deprecated))

If you want to use port 587:
Port 587 with SSL not ticked (UNTICK the checkbox in Email and Conferences module >> Settings >> Email SMTP Server >> SMTPS (deprecated))

Note: The first time the Gateway signs in to Gmail, it will be rejected. Google will send a mail to the gmail address entitled "Review blocked sign-in attempt". This mail will contain a link that you should use to "allow access to less secure apps". After allowing this, the gateway will be able to log in.