Gutenberg override core block

functions.php

<?php

// https://developer.wordpress.org/reference/hooks/render_block/

add_filter( 'render_block', 'foo_core_gallery_filter', 10, 3);

function foo_core_gallery_filter( $block_content, $block ) {

  // use blockName to only affect the desired block
  if( "core/calendar" !== $block['blockName'] ) {
    return $block_content;
  }

	// $block_content contains all the data passed to the block callback function
	// $block contains the serialized block markup
  
  // Here you have all content over the block. Either wrap it in your container or totally replace the content 
  $output = '<h2 class="foo-gallery">';
  $output .= 'GALLERY';
  $output .= '</h2>';

    // return the new content of the block
  return $output;
}