roborourke/gutenberg-frontend-columns.php

## gutenberg-frontend-columns.php
<?php

add_filter( 'the_content', function ( $content ) {
	// Fix layouts HTML.
	libxml_use_internal_errors( true );

	$dom = new \DOMDocument( '1.0', 'UTF-8' );
	$dom->loadHTML( utf8_decode( $content ) );

	$xpath = new \DOMXPath( $dom );

	$layout_classes = apply_filters( 'gutenberg_layout_classes', [ 'layout-main', 'layout-sidebar' ] );

	foreach ( $layout_classes as $layout ) {
		// Get first item with a matching class name inside a div.
		$columns    = $xpath->query( "//div/*[contains(concat(' ', @class, ' '), ' $layout ')][1]" );

		foreach ( $columns as $column ) {
			// Get all items with the same layout class within the same div.
			$container    = $column->parentNode;
			$column_items = $xpath->query( "//*[contains(concat(' ', @class, ' '), ' $layout ')]", $container );

			// Create a new container element with the layout class name.
			$column_node = $dom->createElement( 'div' );
			$column_node->setAttribute( 'class', $layout );

			// Move all the items with the layout class into the new container.
			foreach ( $column_items as $item ) {
				// Remove layout class.
				$item->setAttribute( 'class', trim( str_replace( $layout, '', (string) $item->getAttribute( 'class' ) ) ) );
				$column_node->appendChild( $item );
			}

			// Append the new container with the now nested layout nodes to the parent.
			$container->appendChild( $column_node );
		}
	}

	$content = $dom->saveHTML();

	// Remove saveHTML generated output.
	$content = fix_content( $content );

	libxml_clear_errors();

	return $content;
}, 10 );

function fix_content( $html ) {
	return trim( preg_replace( '/^<!DOCTYPE.+?>/', '', str_replace( array( '<html>', '</html>', '<head>', '</head>', '<body>', '</body>' ), '', $html ) ) );
}
	<?php

	add_filter( 'the_content', function ( $content ) {
	// Fix layouts HTML.
	libxml_use_internal_errors( true );

	$dom = new \DOMDocument( '1.0', 'UTF-8' );
	$dom->loadHTML( utf8_decode( $content ) );

	$xpath = new \DOMXPath( $dom );

	$layout_classes = apply_filters( 'gutenberg_layout_classes', [ 'layout-main', 'layout-sidebar' ] );

	foreach ( $layout_classes as $layout ) {
	// Get first item with a matching class name inside a div.
	$columns = $xpath->query( "//div/*[contains(concat(' ', @class, ' '), ' $layout ')][1]" );

	foreach ( $columns as $column ) {
	// Get all items with the same layout class within the same div.
	$container = $column->parentNode;
	$column_items = $xpath->query( "//*[contains(concat(' ', @class, ' '), ' $layout ')]", $container );

	// Create a new container element with the layout class name.
	$column_node = $dom->createElement( 'div' );
	$column_node->setAttribute( 'class', $layout );

	// Move all the items with the layout class into the new container.
	foreach ( $column_items as $item ) {
	// Remove layout class.
	$item->setAttribute( 'class', trim( str_replace( $layout, '', (string) $item->getAttribute( 'class' ) ) ) );
	$column_node->appendChild( $item );
	}

	// Append the new container with the now nested layout nodes to the parent.
	$container->appendChild( $column_node );
	}
	}

	$content = $dom->saveHTML();

	// Remove saveHTML generated output.
	$content = fix_content( $content );

	libxml_clear_errors();

	return $content;
	}, 10 );

	function fix_content( $html ) {
	return trim( preg_replace( '/^<!DOCTYPE.+?>/', '', str_replace( array( '<html>', '</html>', '<head>', '</head>', '<body>', '</body>' ), '', $html ) ) );
	}