You could also add add-parenthesis
the the class list of the Headline Block, and then add the parenthesis manually to the Headline Block itself through this code:
add_filter('generateblocks_dynamic_content_output', function($content, $attributes, $block){
if ( ! empty( $attributes['className'] ) && strpos( $attributes['className'], 'add-parenthesis' ) !== false ) {
$content = '(' . $content .')';
}
return $content;
}, 10, 3);
Adding PHP: https://docs.generatepress.com/article/adding-php/#code-snippets