I would like to be able to set a lifecycle policy for my GCP bucket that expires entires by when they were last used.

Deleting old items is beneficial because it means that the storage does not constantly expand. However, a basic "delete items by created age" policy might delete entries that are still used. Expiring entries by their unused date would be beneficial, because it's possible that even if an entry is old, it might still be used. This would avoid cache misses.

As I understand, such a policy is possible with a daysSinceCustomTime lifecycle condition, but it requires that the Custom-Time field is set.

Would it be possible for gcp-gradle-build-cache to set Custom-Time when an entry is saved, and successfully read?

https://cloud.google.com/java/docs/reference/google-cloud-storage/latest/com.google.cloud.storage.Blob.Builder#com_google_cloud_storage_Blob_Builder_setCustomTimeOffsetDateTime_java_time_OffsetDateTime_