How to delete a datafile if the father-tablespace does not exists?
The TS associated with your file is 'BGBA_GAL126344', but you are issuing your DROP TABLESPACE on 'GAL126344'.

'BGBA_GAL126344' != 'GAL126344'

Why were you trying to DROP USER?  Neither files nor tablespaces are owned by users.

Why do you repeatedly issue the same failed DROP TABLESPACE command?

FWIW, in 20+ years of working as an Oracle DBA, I have never heard the term 'father-tablespace'.  I think the more correct term would be a file's 'associated tablespace'.