Specifically for gmail you will need to allow “Less secure applications”:
Note for GMail users: SGPro can sometimes be flagged as a "less secure application". In order to use gmail with SGPro, you might need to go here and allow less secure applications access to your address: https://www.google.com/settings/security/lesssecureapps
You should be getting back more than “Test Fails” unless something is highly misconfigured (wrong SMTP server/port)
I have gmail notifications working by setting up to accept “less secure applications” but I have been notified by Google that they will no longer support “less secure applications” after May 30, 2022. All apps will need to support “Sign in with Google” Are there plans to update SGP to allow gmail users to continue to receive notifications?