Lead Developer
Your functions.php file should only have PHP in it.
Your style.css file is where you should add CSS.
Adding CSS: http://generatepress.com/knowledgebase/adding-css/
Adding PHP: http://generatepress.com/knowledgebase/adding-php-functions/
To fix this, access your site using FTP or your hosting control panel File Manager.
Navigate to wp-content/themes/generatepress_child and open the functions.php file.
Remove the CSS you added, and save.
If you’re not comfortable with doing that, try giving your hosting a call, they should be able to remove the problem code and restore your website.
Let me know if you need more info 🙂