Hi there,
Thanks for writing in.
It’s not currently possible, I checked the skin builder and there is no option for Sold out option. I searched some workaround and there is no available as of now.
How about have two grids, first grid for sale, and second grid for sold, then you can use a different skin for each grid (the skin with a layer saying sold.
Then use WP_Query to filter the grid result for sold items, like this
function my_query_args($query_args, $grid_name) {
if ($grid_name == 'My grid name') {
// all query parameters can be modified (https://codex.wordpress.org/Class_Reference/WP_Query)
$query_args['posts_per_page'] = 2;
}
return $query_args;
}
add_filter('tg_wp_query_args', 'my_query_args', 10, 2);
It’s from here https://theme-one.com/docs/the-grid/
And you can implement this query args https://tarei.me/2014/02/03/build-custom-wp-query-loop-getting-woocommerce-stock-products/ for out-of-stock filtering.
This is a bit complex so you may want to ask help from a developer. This is just an idea if you wish to go in that direction (2 separate grid) 
Thanks!