We do not recommend to use this method because of:
login issues, location based issues or similar problems which could occur
GMail has a limit of 500 recipients a day
Guide for Mail Server setup in GMail (free account)
Configure an outgoing mail server
Check your configuration for “Less secure apps”
Check your “Captcha enable” configuration
In case 2FA is used make sure to set a “Application specific” password
Go back to your Confluence Cloud instance and navigate to the mail page configuration in the administration to configure the SMTP Server
Configuring GMail SMTP Server
Server hostname: smtp.gmail.com
Server port: 465 - here you are using port for SSL
Username: username of your Gmail account
Password: password of your Gmail account