Add gutenberg blocks to WordPress API as json.

test.php

<?php

add_action( 'rest_api_init', function () {

	$types = get_post_types(
		[
			'show_in_rest' => true,
		],
		'names'
	);

	register_rest_field( $types,
		'has_blocks',
		array(
			'get_callback'    => function ( $object ) {
				return has_blocks( $object['id'] );
			},
			'update_callback' => null,
			'schema'          => array(
				'description' => __( 'Has blocks.' ),
				'type'        => 'boolean'
			),
		)
	);

	register_rest_field( $types,
		'blocks',
		array(
			'get_callback'    => function ( $object ) {
				$blocks = parse_blocks( $object['content']['raw'] );
				$output = [];
				foreach ( $blocks as $block ) {
					if ( $block['blockName'] ) {
						$block['rendered'] = render_block( $block );
						$output[]   = $block;

					}
				}

				return $output;
			},
			'update_callback' => null,
			'schema'          => array(
				'description' => __( 'Blocks.' ),
				'type'        => 'object'
			),
		)
	);
} );