To add the item meta information in the google calendar event description.

functions.php

/**
 * Adding the meta data to the Google Event Description
 *
 * @param string $description Event Description.
 * @param obj    $app Objection of all information for the calculation.
 */
function bkap_google_event_description_callback( $description, $app ) {

	$item_id             = $app->item_id;
	$item                = new WC_Order_Item_Product( $item_id );
	$item_data           = $item->get_data();
	$formatted_meta_data = $item->get_formatted_meta_data( '_', true );

	$name           = get_option( 'book_item-meta-date' );
	$name           = ( '' == $name ) ? __( 'Start Date', 'woocommerce-booking' ) : $name;
	$name_checkout  = get_option( 'checkout_item-meta-date' );
	$name_checkout  = ( '' == $name_checkout ) ? __( 'End Date', 'woocommerce-booking' ) : $name_checkout;
	$name_time_slot = get_option( 'book_item-meta-time' );
	$name_time_slot = ( '' == $name_time_slot ) ? __( 'Booking Time', 'woocommerce-booking' ) : $name_time_slot;

	$labels = array( "Geburtsdatum (Geburtsjahr: auf aktuelles Jahr klicken und ändern!)" ); // BirthDate Label .

	$extra_data = '';
	foreach ( $formatted_meta_data as $key => $value ) {
		if ( in_array( $value->key, $labels ) ) { // Check only for birthdate data.
			$extra_data .= '<br>' .  $value->display_key . ' : ' . $value->display_value;
		}
	}

	$description = str_replace(
		array( 'ADDITIONAL_DATA' ),
		array( $extra_data ),
		$description
	);

	return $description;
}
add_filter( 'bkap_google_event_description', 'bkap_google_event_description_callback', 10, 2 );