I was able to move the category to the left side of the gallery (as one column of vertical list of thumbnails) on the main page of my gallery.
I'm not quite sure how to do it, but I duplicated function starttable($width = '-1', $title = '', $title_colspan = '1') on theme.php, changed the table class name, and renamed the function, so that my category has its own table, separated from others.
How do I display category as "one column of vertical list of thumbnails on the left side of the gallery" not only on the main page, but also everywhere on the gallery (Album List View, Thumbnail View, and Image View)?