If you want change dl lists to ul lists on all galleries, not only on those that request this via extra attributes, you can hook into the post_gallery filter that gets run at the beginning of the gallery_shortcode function. There you can override and set default values for the attributes.

The final output is not filtered, but I guess it should be possible to remove the default shortcode handler for gallery, and add your own function that wraps gallery_shortcode() but adds final processing at the end. Or try hooking into wp_get_attachment_link, as Rarst suggests.

If you want change dl lists to ul lists on all galleries, not only on those that request this via extra attributes, you can hook into the post_gallery filter that gets run at the beginning of the gallery_shortcode function. There you can override and set default values for the attributes.

The final output is not filtered, but I guess it should be possible to remove the default shortcode handler for gallery, and add your own function that wraps gallery_shortcode() but adds final processing at the end. Or try hooking into wp_get_attachment_link, as Rarst suggests.