Hi Parita,
Found the solution.
Pass the Value in itab1 as below, it will give your the proper format of header.
1 Column
gw_tab1-line = text-004.
append gw_tab1 to itab1.
2 Column
gw_tab1-line = text-005.
append gw_tab1 to itab1.
...Like that you append your columns name into the itab1 table and then pass the same to FM.
You will get the Correct Header in file.
Regards.
Praveer.