Hi,
I've never seen that before - a discussion posting with just source code and no questions!
Are we supposed to use ESP? 
Hold on, I just saw it:
ASSIGN COMPONENT v_fieldvalue OF STRUCTURE <gfs_line>
TO <fs_field1>. .......................... (SY-SUBRC = 4)
This would be caused by <gfs_line> not being assigned, OR by v_fieldvalue not being a valid fieldname of structure <gfs_line>.
It would be easy to see which one it is during debug. Or you could check the text in the dump.
cheers
Paul Bakker