I could only figure out 2 possible ways:

1. function / code duplication
2. Regular expression

If you use [gallery] shortcode without setting the columns value, the default is set to 3. Some WordPress theme can either fit 3 or more column per row or less. Below is an example to force [gallery] columns to 2 to any post or page that uses [gallery] shortcode:

function gallery_columns($content){
	$columns = 2;
	$pattern = array(
		'/(\[gallery(.*?)columns="([0-9])"(.*?)\])/ie',
		'/(\[gallery\])/ie',
		'/(\[gallery(.*?)\])/ie'
	);
	$replace = 'stripslashes(strstr("\1", "columns=\"$columns\"") ? "\1" : "[gallery \2 \4 columns=\"$columns\"]")';

	return preg_replace($pattern, $replace, $content);
}

add_filter('the_content', 'gallery_columns');

$columns can be set to any numeric value. If [gallery] columns is set to 0, no row breaks will be included.

There are probably some hidden drawbacks to the method above. Feel free to share if you have any other solution.

add_filter('gallery_style',
	create_function(
		'$css',
		'return preg_replace("#<style type=\'text/css\'>(.*?)</style>#s", "", $css);'
		)
	);

And these are the classes for your stylesheet:

.gallery {}
.gallery-item {}
.gallery-icon {}
.gallery-caption {}