Hi ,
Very late to reply.
Pass values in Filednames parameter.
Sample :
LOOP AT it_fcat_pnl INTO lw_fldcat.
CLEAR :wa_field_name.
wa_field_name-field_name = lw_fldcat-coltext.
APPEND wa_field_name TO i_field_name.
CLEAR : lw_fldcat.
ENDLOOP.
CALL FUNCTION 'GUI_DOWNLOAD'
EXPORTING
filename = p_file
filetype = 'ASC'
* APPEND = ' '
write_field_separator = 'X'
* IMPORTING
* FILELENGTH =
TABLES
data_tab = <lfs_table>
fieldnames = i_field_name
EXCEPTIONS
file_write_error = 1
no_batch = 2
gui_refuse_filetransfer = 3
invalid_type = 4
no_authority = 5
unknown_error = 6
header_not_allowed = 7
separator_not_allowed = 8
filesize_not_allowed = 9
header_too_long = 10
dp_error_create = 11
dp_error_send = 12
dp_error_write = 13
unknown_dp_error = 14
access_denied = 15
dp_out_of_memory = 16
disk_full = 17
dp_timeout = 18
file_not_found = 19
dataprovider_exception = 20
control_flush_error = 21
OTHERS = 22
.
IF sy-subrc <> 0.
MESSAGE ID sy-msgid TYPE sy-msgty NUMBER sy-msgno WITH sy-msgv1 sy-msgv2 sy-msgv3 sy-msgv4.
ENDIF.
It will surely help.