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.
After changing your BigCommerce store currency, the change will not be reflected in Gelato. Any new products will still be created in the original store currency.
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?.