Solution 1:

There's plenty of tutorials that you can follow to configure a FTP/SFTP server on Google Cloud, keep in mind that this is not Google documentation.

https://cloudinfrastructureservices.co.uk/how-to-setup-filezilla-secure-ftp-server-on-google-cloud-server-2016/ https://www.onepagezen.com/google-cloud-ftp-filezilla-quick-start

Regarding your current configuration, I agree with the previous users, you should also open the ports on the Firewall of CentOS, and apply the tag you defined on the Firewall Rule on the CentOS Instance.