Where to put meta Keys

First of all you have an if statement that is always false in your save function.

I don’t see any input called wporg_field anywhere in your meta box, so most probably there is no such field – so that condition is false and no meta field is saved.

Another problem is that you make wrong assumptions on how checkboxes work. If checkbox is not checked, then it won’t be sent in POST array.

Here’s a fixed version – it should work:

function wporg_save_postdata($post_id)
{
    if ( array_key_exists('include_in_image_gallery', $_POST) ) {
        update_post_meta(
            $post_id,
            '_wporg_meta_key',
            $_POST['include_in_image_gallery']
        );
    } else {
        delete_post_meta($post_id, '_wporg_meta_key');
    }
}