How can I add a free gift option to my woocommerce site? It should work like this:

The customer can choose from a variety of gifts. The gifts are obviously free, but they should have a certain value (e.g. this gift would cost 20$ but it is obviously free). I need this because the customer is allowed to choose gifts worth 10% of the cart value:

If the customer buys some products for 500$ He can choose gifts worth 50$.

How can I achieve this? There is one PlugIn called “Free Gifts for WooCommerce”.
https://ithemelandco.com/plugins/free-gifts-for-woocommerce/
But I don’t know if this works. Do you have some ideas?