Example Migrated from Codex:

Hook into the gallery shortcode and replace its output with your own.

add_filter( 'post_gallery', 'my_gallery_shortcode', 10, 3 );

function my_gallery_shortcode( $output = '', $atts = null, $instance = null ) {
	$return = $output; // fallback

	// retrieve content of your own gallery function
	$my_result = get_my_gallery_content( $atts );

	// boolean false = empty, see http://php.net/empty
	if( !empty( $my_result ) ) {
		$return = $my_result;
	}

	return $return;
}