wp-gutenberg-block-template/index.php

<?php
add_action( 'init', function() {
	$block_namespace = 'namespace';  // TODO
	$block_slug =  'slug';           // TODO

	if ( ! function_exists( 'register_block_type' ) ) {
		return;
	}

	$dir = get_template_directory() . "/blocks/$block_slug";
	$uri = get_template_directory_uri() . "/blocks/$block_slug";

	$asset_file = include( get_template_directory() . "/blocks/$block_slug/build/index.asset.php");

	$index_js = 'build/index.js';
	wp_register_script(
		"{$block_slug}-block-editor",
		"$uri/$index_js",
		$asset_file['dependencies'],
		$asset_file['version']
	);

	$editor_css = 'build/index.css';
	wp_register_style(
		"{$block_slug}-block-editor",
		"$uri/$editor_css",
		array(),
		filemtime( "$dir/$editor_css" )
	);

	$style_css = 'build/style-index.css';
	wp_register_style(
		"{$block_slug}-block",
		"$uri/$style_css",
		array(),
		filemtime( "$dir/$style_css" )
	);

	register_block_type( "$block_namespace/$block_slug", array(
		'editor_script' => "{$block_slug}-block-editor",
		'editor_style'  => "{$block_slug}-block-editor",
		'style'         => "{$block_slug}-block",
	) );
} );