You can easily change the store currency in Squarespace from your Squarespace store. You can read more about how you can do so in Squarespace's Help Center Article Changing your store's currency.
After changing your Squarespace 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?.