Below is a select statement you can use to select the data from the SYSMAN views.
ceil(sizeb/1024/1024/1024) as allocated_gb,
ceil(usedb/1024/1024/1024) as used_gb,
ceil(freeb/1024/1024/1024) as free_gb
order by 1 asc
Note: If you are going to use another account other than SYSMAN to select you will need to have the following privilege.
grant select on mgmt$storage_report_data to <username>;
grant exempt access policy to <username>;