Before making a GET order request, a user should have an already CREATED order, which can be paid or canceled. To CREATE ORDER, please follow the instructions here https://developer.coingate.com/docs/create-order
After creating an order, you will get an ORDER ID. This ID will be used for GET ORDER requests.
Please, take into account that before creating requests within the CoinGate system, the user should have an authentication token. To have it, please follow the instructions here https://developer.coingate.com/docs/api-authentication
pay_currency, pay_amount, expire_at, payment_address are only sent when the customer chooses the currency with which he is going to pay for the invoice.