Thanks for getting in touch. If you wish to use a third-party gateway to process in-person payments, you will need to use an external card reader to process the payments for you. You would need to speak with Pinwheel about renting or buying a terminal that can process in-person payments using their gateway and then set up a custom payment method in the Shopify POS app.
Opting to use a custom payment method for in-person payments means that the order and most order data (e.g. order number, customer information, order total) will still be logged in the Orders section of your admin, but the payment itself will be processed by the third-party gateway. If you wish to test ahead of time that the gateway is working, I'd recommend putting through a small order via the Shopify POS using the custom payment option and third-party terminal before you use it in your store with real customers.
I hope this explains things but please let me know if you have any further queries.
Victor | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog