Your application stores files on Cloud Storage by using the Standard Storage class.

Create a retention policy on the storage bucket of 30 days, and lock the bucket by using a retention policy lock.

Enable object versioning on the storage bucket and add lifecycle rules to expire non-current versions after 30 days

Create an object lifecycle on the storage bucket to change the storage class to Archive Storage for objects with an age over 30 days.

Create a cron job in Cloud Scheduler to call a Cloud Functions instance every day to delete files older than 30 days.