hack to inject better sizes attribute into WordPress Gutenberg Gallery Block

wp-blockgallery-fixsizes.php

<?php
/* Not really recommended, very dangerous. Will change entities.
 * Adjust the xpath query & sizes to your needs.
 */

add_filter('the_content', function ($content) {
    try {
        $preamble = '<?xml encoding="utf-8" ? >';
        $doc = new DOMDocument();
        $doc->preserveWhiteSpace = true;
        $doc->substituteEntities = false;
        $doc->loadHTML($preamble.$content, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD | LIBXML_NOWARNING | LIBXML_NOERROR);
        $xpath = new DOMXpath($doc);
        $imgs = $xpath->query('//ul[contains(@class, "wp-block-gallery") and contains(@class, "columns-2")]//figure//img'); // adjust
        if (!$imgs) {
            // better not mess around
            return $content;
        }
        else {
            foreach ($imgs as $img) {
                $img->setAttribute('sizes', '(max-width: 781px) 50vw, 344px'); // adjust
            }
        }
        return str_replace($preamble, '', $doc->saveHTML());
    }
    catch (Exception $e) {
        // give up
        return $content;
    }
});