The Google drive integration allows to take backup weekly or daily. It is preferred to be able to delete old backups.
It seems like a good idea that the deletion process (Say for files older than 8 days) should be done by ERPNext after a successful backup. This would ensure that in case of failure of backups for some time older backups are not deleted.
I plan to implement this since deletion of files in Google Drive seems easy once you have authenticated yourself. However I was not able to find the code base in git-hub which is responsible for GDrive backup in v12 master.