You can easily change the store currency in BigCommerce from your BigCommerce store. You can read more about how you can do so in BigCommerce's Help Center Article Managing Currencies.
Please remember to go back to your Gelato dashboard and sync the store to ensure all updates are synced and reflected on the Gelato end. Otherwise, the currency will not be updated.
It may take some time after syncing before WiX updates are reflected in the Gelato Dashboard.
You can change the billing currency of your Gelato account as well. Read more on how to do so in our article How can I change the currency in my Gelato account?