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.

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. If you skip this step, Gelato will be creating new products using the wrong currency.
It may take a few minutes before changes are reflected on the Gelato end after syncing the store.

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?