Fill a GTT with external data each session
I have to import an external csv file into a table in database each
  day.
  
  ...
  
  Additionally, It is much appreciated if the importing could be done
  entirely without the need for GTTs and loading a file straight into
  the destination table.


And that is what SQL*Loader was made for.

SQL&gt; create table employee
(
  id integer,
  name varchar2(10),
  dept varchar2(15),
  salary integer
);

$ cat employee.csv
100,Thomas,Sales,5000
200,Jason,Technology,5500
300,Mayla,Technology,7000
400,Nisha,Marketing,9500
500,Randy,Technology,6000
501,Ritu,Accounting,5400

$ cat load.ctl
load data
 infile 'employee.csv'
 append
 into table employee
 fields terminated by ","
 ( id, name, dept, salary )


Load data into empty table:

$ sqlldr user/password@database control=load.ctl

SQL&gt; select * from employee;

        ID NAME       DEPT                SALARY
---------- ---------- --------------- ----------
       100 Thomas     Sales                 5000
       200 Jason      Technology            5500
       300 Mayla      Technology            7000
       400 Nisha      Marketing             9500
       500 Randy      Technology            6000
       501 Ritu       Accounting            5400


Append data (notice the append keyword in load.ctl):

$ sqlldr user/password@database control=load.ctl

SQL&gt; select * from employee;

        ID NAME       DEPT                SALARY
---------- ---------- --------------- ----------
       100 Thomas     Sales                 5000
       200 Jason      Technology            5500
       300 Mayla      Technology            7000
       400 Nisha      Marketing             9500
       500 Randy      Technology            6000
       501 Ritu       Accounting            5400
       100 Thomas     Sales                 5000
       200 Jason      Technology            5500
       300 Mayla      Technology            7000
       400 Nisha      Marketing             9500
       500 Randy      Technology            6000
       501 Ritu       Accounting            5400