How to convert Bytes to GB using sql query
you can use below function :

create or replace function format_size(p_size IN NUMBER) 
return VARCHAR2
IS
  v_i number;
  type array_t is varray(3) of varchar2(10);
  v_array array_t := array_t(' Bytes', ' KB', ' MB',' GB');
BEGIN
  v_i := floor(log(1024, p_size));
  return to_char(round(p_size/power(1024, v_i), 2))||v_array(v_i+1);
END;