randomize_gallery_block.php

<?php

function randomize_block_core_gallery( $parsed_block ) {
	if ( 'core/gallery' === $parsed_block['blockName'] && isset( $parsed_block['attrs']['className'] ) && 'random' === $parsed_block['attrs']['className'] ) {
		shuffle( $parsed_block['innerBlocks'] );
	}

	return $parsed_block;
}
add_filter( 'render_block_data', 'randomize_block_core_gallery' );