Yes, you can add a new variation to an already published product on your WooCommerce store.
Add the new Variation in WooCommerce
First, you will have to add the new variation in WooCommerce. You can read more on how to do so in the WooCommerce Help Center article Variable Product.
Sync products in Gelato
Once the new variation is created in WooCommerce, navigate to the Stores section and select your WooCommerce store. Click on the "Sync products" button is located on the top right of the page. After a while, you will see a new variant as "Not connected".
Connect the new variation in Gelato
You will now be able to connect the new variation to a product in Gelato as shown here.