a ver yo lo trabajo de esta manera

CALL FUNCTION 'GUI_DOWNLOAD'
EXPORTING
FILENAME = ls_filename
FILETYPE = 'ASC'
TABLES
DATA_TAB = data_tab
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.