Reply To: Get File Names in Email
$entry variable is the Gravity Forms entry data. https://docs.gravityforms.com/entry-object/
syncs3_put_object_file_path filter needs a full path to be returned, which is a path relative to the bucket. Meaning, if you want your file to be added to a folder within your bucket, it will need to be part of the path.
In your example code, there would be no folders. Instead, the file would be saved directly under the bucket, with a filename that looks something like
entry-1-filename.txt. If you need folders, add the structure to the beginning of your path.