this command gsutil ls -a gs://
Are you sure you turned on object version on your bucket?
the -a flag only works if versioning is turned on; only for files in the bucket, not the bucket itself.
here a link to the details: https://cloud.google.com/storage/docs/gsutil/commands/ls#options
please clarify additional information, happy to help find out where the information went wrong.
I guess versioning works on files/folders within the bucket, not on the bucket itself. If you have removed the bucket it will not be available even if versioning is on.
But for files within the bucket, please re-verify that versioning is on by gsutil versioning get gs://