<?php
function modified_post_gallery( $blank = NULL, $attr )
{
echo '<pre>';
print_r( $attr );
echo '</pre>';
}
add_filter( 'post_gallery', 'modified_post_gallery', 10, 2);
?>
In media.php it shows the filter like this:
// Allow plugins/themes to override the default gallery template.
$output = apply_filters('post_gallery', '', $attr);
Its passing two variables not one and $attr is on the second variable.