This was my eventual implementation. It adds brand and all the gallery images.
add_filter( 'woocommerce_structured_data_product', function( $markup, $product ) {
// add Brand
if($brand = $product->get_attribute( 'pa_brand' )) {
$markup['brand'] = $brand;
}
// product gallery images
if($attachment_ids = $product->get_gallery_image_ids()) {
$gallery_images = array($markup['image']);
foreach( $attachment_ids as $attachment_id ) {
$gallery_images[] = wp_get_attachment_url( $attachment_id );
}
$markup['image'] = $gallery_images;
}
return $markup;
}, 10, 2 );
If others find it useful