I'm not privy to Matt's code, but here's how I would do it.
First, http://ma.tt/category/gallery/ looks to be a category template. That means that it's probably a file named something like
category-14.php, where 14 is the ID of the "gallery" category. Here's the gist of what's in that category template:
<?php add_filter('gallery_style',create_function('$a','return "";')) ?>
<?php while (have_posts()) : the_post(); ?>
<?php $attachments = get_children(array('post_parent' => get_the_ID(), 'post_type' => 'attachment', 'post_mime_type' => 'image', 'orderby' => 'menu_order'));
if ( ! is_array($attachments) ) continue;
$count = count($attachments);
$first_attachment = array_shift($attachments); ?>
<h2><a href="<?php the_permalink() ?>"><?php echo single_cat_title(); ?>: <?php the_time('Y-m-d') ?></a></h2>
<a href="<?php the_permalink() ?>">
<?php echo wp_get_attachment_image($first_attachment->ID); ?>
<?php the_excerpt() ?>
<p>This album contains <strong><?php echo $count ?> items</strong>.</p>
<?php endwhile; ?>
As with all templates, this one uses the Loop to print out posts. So there is the standard Loop while loop, within which we get the first attachment info for each post and print the excerpt of the post as the description.
'gallery_style' filter function just keeps the gallery CSS from showing up.