Gutenberg gallery hook errors

I try to modify the gallery block of Gutenberg and I use the following snippet

export function defaultColumnsNumber( attributes ) {
    console.log(attributes.images.length)
    return Math.min( 3, attributes.images.length );
}

function modifyGalleryMarkup( element, blockType, attributes ) {

    if ( blockType.name !== 'core/gallery' ) {
        return element;
    }

    const { images, columns = defaultColumnsNumber( attributes ), imageCrop, linkTo } = attributes;

    console.log(attributes)
    var newElement = wp.element.createElement(
        'ul',
        {
            'className': `lightbox columns-${ columns } ${ imageCrop ? 'is-cropped' : '' }`,
        },
        attributes.images.map(
            function( image ) {
                return wp.element.createElement(
                    'li',
                    {
                        'className': 'blocks-gallery-item my-gallery-item',
                    },
                    wp.element.createElement(
                        'img',
                        {
                            'src': image.url,
                        }
                    )
                )
            }
        )
    )

    return newElement
}

wp.hooks.addFilter(
    'blocks.getSaveElement',
    'plugin/modify-gallery',
    modifyGalleryMarkup
);

I can save it but on page reload I get the following error in console

Block validation failed for `core/gallery`

Expected:

<ul class="lightbox columns-0 is-cropped"></ul>

Actual:

<ul class="lightbox columns-3 is-cropped">< ....

what I’m missing in my snippet?

Seems that when you modify the existing block’s HTML, then it is no longer being validated.
Instead, you might try to create your own block. Look here is a good approach and here too.