Shailaja,
I don't believe GUI_DOWNLOAD should be used for BSP. I believe there is a way to upload and download files via BSP, however I personally haven't done that before. I would recommend searching download file in the BSP forum.
I did a quick search and found the following weblogs written by Thomas Jung:
BSP Download to Excel in Unicode Format
Creating a BSP Extension for Downloading a Table
I think you should be able to adapt his example hopefully for the PCUI.
Good luck,
Stephen