WP_HTML_Tag_Processor add_class

wp-tag-processor-add-class.php

<?php
$hook = "render_block_someBlock";
    
add_filter( $hook, function($block_content, $block, $instance) {
    $selector = explode(',', $blockInfo['selector'] ?? '');

    $content = new WP_HTML_Tag_Processor( $block_content );

    $content->next_tag( $selector );
    $content->add_class( 'g-rich-text' );

    $block_content = (string) $content;

    return $block_content;
}, 10, 3 );