// add a filter to the gallery shortcode
add_filter( 'shortcode_atts_gallery', 'filter_gallery_by_tag',10,3 );

function filter_gallery_by_tag( $out, $pairs, $atts ) {

	$post = get_post();

	if ( !$post || !isset( $atts['tag_id'] ) ) {
	 	return $out;
	}

	// check if the current post has the tag id
	if ( has_term( $atts['tag_id'], 'post_tag', $post ) ) {

		$posts_per_page = ( isset( $atts['posts_per_page'] ) ) ? intval( $posts_per_page ) : 6;

		$args = array(
			'posts_per_page' => $posts_per_page,
			'fields' => 'ids',
			'tag_id' => $atts['tag_id'],
			'meta_query' => array(
				array( 'key' => '_thumbnail_id' )
			),
		);

		// get posts with post thumbnails that have the same tag id
		$related_posts = get_posts( $args );


		if ( $related_posts ) {

			$thumb_ids = array();

			foreach ( $related_posts as $related ) {
				$thumb_ids[] = get_post_thumbnail_id( $related );
			}

			$out['include'] = $thumb_ids;
		}
	}

	return $out;
}