Hola Me gustaría saber como poner un fondo para pc y una foto para movil

The additional CSS class field is a WP core feature, if you still can't save the class, you'll need to debug with WP.

For now, let's use GB container's ID to write CSS:

 @media (max-width: 768px) {
    .gb-container-95296705:before {
        background-image: url('http://videnteatiendopersonalmente.com/wp-content/uploads/2022/03/AdobeStock_2558769853-compressed.jpg');
    }
}

The URL should be the image that you want to use on mobile, make sure it's different from the one you are using on desktop.