Note
Cloud Source Repositories are intended to store only the source code for your application and not user or personal data. Do not store any Core App Engine End User Data (as defined in your License Agreement) in a Cloud Source Repository. To use a hosted Git repository with a Cloud Source Repository, you must first open an account with GitHub or Bitbucket (independent companies separate from Google). If you push source code to a Cloud Source Repository, Google will make a copy of this data which will be hosted in the United States.
Are they saying it is not secure?
https://cloud.google.com/tools/cloud-repositories/docs/
Note Cloud Source Repositories are intended to store only the source code for your application and not user or personal data. Do not store any Core App Engine End User Data (as defined in your License Agreement) in a Cloud Source Repository. To use a hosted Git repository with a Cloud Source Repository, you must first open an account with GitHub or Bitbucket (independent companies separate from Google). If you push source code to a Cloud Source Repository, Google will make a copy of this data which will be hosted in the United States.