function my_custom_admin_page() {
    add_menu_page(
        'Custom Page with Gutenberg Editor',
        'Custom Page',
        'manage_options',
        'my-custom-page',
        'my_custom_page_callback',
        'dashicons-edit',
        20
    );
}
add_action('admin_menu', 'my_custom_admin_page');

function my_custom_page_callback() {
    ?>
    <div id="my-custom-editor"></div>
    <script type="text/javascript">
        (function() {
            var el = document.getElementById('my-custom-editor');
            wp.blocks.getBlockTypes();
            wp.element.render(
                wp.element.createElement(
                    wp.blockEditor.BlockEditorProvider,
                    {},
                    wp.element.createElement(
                        wp.blockEditor.BlockList,
                        {
                            blocks: wp.data.select('core/block-editor').getBlocks()
                        }
                    )
                ),
                el
            );
        })();
    </script>
    <?php
}